The popular idea that " light puts out the fire," is so fixed, j that probably no conclusions drawn from actual experiment are likely to disturb it, especially if they be averse to the notion. It is a matter of daily experience, people say, that, if the fire is nearly out, and you put a screen before it, or draw down the blind, or close the window shutters, it will immediately begin to revive. It is generally forgotten that a fire that looks dull, or " out," in a well-lighted room, will appear to be in tolerable condition in the same room when darkened. It only requires to be " put together " to make it burn up, and it might have been done so just as well in the light Experiments on this subject are not easy to make, on account of the many disturbing causes. In an old volume of the " Annals of Philosophy," is an account of some experiments by Dr. M'Keever, who took two portions of green wax taper, each weighing 10 grains, and ignited both at the same moment. One piece was placed in a dark room, at 67 Fah.; the other was exposed to broad sunshine, at 78 Fah. t c -4. ( The taper in sunshine lost........8 J grains. In five mmutes -j darkened room lost.. 9i " The taper, divided into inches, was also burnt in the colored portions of the solar spectrum, when it was found that the time required to burn two inches of taper varied as follows: Minutes. Seconds. In the red ray it took.............. 8 0 In the green it took................8 20 In the violet it took.................8 39 In the verge of the violet............8 57 The conclusion is, that the solar rays, in proportion to their intensity, have the power of retarding, to a considerable extent, the process of combustion ; and it is supposed that the chemical rays act, in some way, on the portion of oxygen about to Gombine with tlie fuel, so as to delay, if not prevent combination. Supposing, in the experiments, the taper was so uniform, that one inch contained precisely the same quantity of matter as another inch, the time occupied in burning was too short to j ustify so important a conclusion as Dr. M'Keever arrived at, whether the results were taken by measure or by weight. Every one engaged in photometrical observations must be aware of the difficulty of getting rid ol disturbing causes and perplexing results. In comparing candles of the same make, the light is aflfected, both in quantity and economy, by a number of small circumstances, such as the warmth of the room, the existence of slight currents of air, the extent to which the wick curls over in burning, and so on. In testing the quality of gas, the standard candle defined by act of Parliament is ? sperm candle of six to the pound, burning at the rate of 130 grains per hour. From such a standard we get the terms "twelve-candle gas," " fourteen-candle gas," etc. Mr. Sugg, in his " Gas Manipulation," has pointed out some of the dUBculties in obtaining a uniform standard candle. The wick does not always contain the same number of strands; they are not all twisted to the same degree of hardness ; the so-called sperm may vary in composition—one candle containing a little more wax than another, or variable quantities of starine or of paraffine ; the candle may have been kept in store a long or a short time ; the temperature of the store room may have varied considerably, and the temperature of the room in which it was burnt may have been high or low. All those circumstances afl'ect the rate of combustion, and the illuminating power of candles, irrespective of the action of light, if such action really exist. I have lately had a good opportunity of testing this action at the works of Price's Patent Candle Company, at Battersea. Um'erthe direction of Mr. Hatclier, the accomplished chemist of the Company, the greatest possible care is taken to insure identity of composition and illuminating power in candles of the same name. There has lately been an extensive series of experiments on the photometrical value ot sperm candles, during which, at my request, Mr. Hatcher was good enough to note the rate of combustion of sach candles in a darkened room, and also in broad daylight, and even in sunshine. In the first observation three hard and three soft candles were burned, each for four hours, in a dark closet. A similar set of candles, taken from one and the same filling, were burned during tlio same time, in open daylight, partly in sunlight. The average consumption per hour of each candle was as follows : Sperm, in the dark..................134 grains " light.................141 " (In the dark..........133 " No. 3 composites j It must be noticed that the temperature in the light was 73, and in the dark 71 ; moreover, in the light there was a much greater motion of the air than in the dark closet. Both these circumstances would operate in producing a larger consumption of candle. In a second trial with No. 8 composites, the results were : In the dark........................140 grains In the light........................134 In a third trial, also with No. 2 composites, the results were : In the dark.........................131 grains In the light........................129 " In these two trials, the flames were protected, as far as 1 possible, from currents of air ; and, in a third trial, the , temperature, both in the light and in the dark, was nearly equal. The fourth trial %vas made on a bright, sunshiny day, with hard sperm candles, which are less affected by variations of temperature than the composites. The results were: In the dark (temperature, 81), 544 grains, or 136 grains per hour. In the light (temperature, 84), 567 grains, or 143 grains per hour, nearly. It is evident that, in this case, the increase of temperature caused by the bright sunsiiine led to an increased consumption of material. It will be seen that, in the first and fourth trials, there is a greater consumption of material in the light than in the dark ; and in the second and third trials, the consumption is greater in the dark than in the light ; but, in any case, the difference is so small—amounting only to from two to seven grains per hour—that it may fairly be referred to accidental circumstances, such as difiirence in temperature, in currents of air, and in the composition and make of the candles ; the final conclusion to which I am led being that the direct light of tlie sun, or the diffused light of day, has no action on the rate of burning or in retarding the combustion of an ordinary candle. Ammonia Powder. The following account of a new explosive material appears in the " Klnische Zsitung," May 19, which gives the " Mili-tar-Wochen-blatt " as its authority : It is now some time since the proprietors of the Nora-Gyttorn Powder Mills obtained a patent in Sweden for the discovery of tlio so-called " ammonia powder," a substance which has hitherto been only employed in a few mining districts, but which otherwise seems wholly unknown. We are, therefore, fully justified in.calling attention to the particular properties of this new explosive material. During the short time that it has been employed, it has won the approval not only of the proprietors of mines, but also of the working miners themselves. Its explosive force may be compared to that of nitro-glycerin, and, consequently, far surpasses that of dynamite. It cannot be exploded by a flam i -r I y sparks, and tlie explosion is afl'ected by a heavy blow from i hammer. Blast holes loaded with this powder are exploded by means of a powerful cap, or, better, by means of a cartridge containing common powder, for this forms a more reliable exploder. Miners who have been obhged to give up the use of nitro-glycerin, cu account of the danger connected witli this powerful explosive agent have a most satisfactory substitute in the ammonia powder, as the danger of using it is so small that it surpassesin safety every other blasting material. One of tlie useful and important properties of this new powder is, that it does not require h Citing in cold weather, whilst nitro-glycerin and dynamite must first of all be warmed, and this has been the cause of many accidents. The price of ammonia powder is the same as that of dynamite. The same paper further adds : According to information we have received, ammonia powder was discovered by the chemist Norrbin. The Oerman " Building News" contains extracts from a report of the Prussian architect Steenke, who makes the following remarks upon the safety of ammonia powder : Experiments were made by fastening a lamp to a pendulum, which was cause ; to oscillate ; gunpowder, gun-cotton, nitro-glycerin, and dynamite all took fire as the fiame passed ever them, but the ammonia powder did not begin to burn till it had been touched by the fiame twenty times. In makmg experiments upon the force of the blow required to explode it, it was found that, with the apparatus employed, where the fall of a weight from 4ft. to 5ft. would explode gunpowder, nitro-glycerin only required lift, to 3ft., dynamite 3fft. to 3ft. fall, wliilst a fall of from 13ft. to 15ft. was necessary to cause the explosion of the ammonia powder. Varnisli for Iron. The following is a method given by Ji. Weiszkopf, of producing upon iron a durable black shining varnish : " Take oil of turpentine, add to it, drop by drop and while stirring, strong sulphuric acid until a sirupy precipitate is quite formed, and no more of it is produced on furthur addition of a drop of acid. The liquid is now repeatedly washed with water, every time refreshed after a good stirring, until the water does not exhibit any more acid reaction on being tested with blue litmus paper. The precipitate is next brought upon a cloth filter, and, after all the v.'ater has run ofi; the sirupy mass is fit for use. This thickisii magma is painted over the iron with a brush ; if it happens to be too stiff, it is previously diluted with some oil of turpentine. Immediately after the iron has been so painted, the paint is burnt in by a gentle heat, and, after cooling, the black surface is rubbed over with a piece of woollen stuff dipped in, and moistened with linseed oil. According to the author, this varnish is not a sinple covering of the surface, hut it is chemically combined with the metal, and does not, therefore, wear ofi' or peel off, as other paints and varnishes do, from iron." POETLANB CEMENT.—Vv'o are in receipt of numerous inquiries relative to Portland cement, where it is made, whether it will make a good miU-dam, how it will ansvfer for concrete pavements, what are all the details of mixing, etc., etc., and lastly, who makes and sells it in this country. We have already devoted considerable space to this subject. To answer all the inquiries put to us in regard to it, we should have to ivrite a work equal in size to Eeid's " Practical Treatise on Portland Cement," published by Henry Carey Baird, 406 Wal nut St., Philadelphia. We recommend those desiring information to get this work, and those who are interested in the sale of this cement to advertise it In the SCBBNTIEIC AJIBRI- CAST. Improvciricnt In Rotary inoIdlnK Outtcrs. Tliis machine is designed to cut all kinds of irregular, circular, or elliptic molding, for which kinds of work it possesses many advantages. These advantages will become apparent upon an examination of the accompanying engravings and a brief description of its construction. The cutter, shown in Fig. 3, is made from a block of steel by first turning it on the outside to the form of the molding required. The inside is next turned out concave, as shown in the engraving, reducing the steel to the requisite thinness, then bored to fit the spindle. Portions are then cut from the outer part of the cup-shaped piece of steel formed by the outside and inside tvirning, leaving radial branches or segments, as shown in the engraving. The cutter is secured to the spindle by a single nut turned down upon a collar which rests upon the flat portion of the interior of the cutter. It is shown thus attached in Fig. 1. The advantages of this form of the cutter are, first, that the concave form of the interior affords ample space for the clearance of the cuttings ; second, facility iu sharpening, as an oil stone of the proper size and form may be readily applied ; and third, the securing of thin beveled edges, whereby a much cleaner and smoother cut can be obtained. The machine on which this cutter is placed. Pig. 1, is constructed as follows : The table is raised or lowered by parallel inclines operated by a hand-wheel and screw. By means of a straight and a cross belt, with fast and loose pulleys on the primary shaft of the machine, a right or left motion is given to the cutter at the will of the operator; the belt shipper being operated by a foot lever, as shown, attached to a longitudinal rock shaft, having at its opposite end a slotted arm, which moves the shipper. The advantages secured iu this combination of machine and cutter are, first, the capability of running the cutter to the right or left by which the work is certain to match ; second, the short time required to set the cutters ; third, perfect freedom in moving the work on the table, as there is but one spindle ; fourth, the sharpening of the cutters on the inside, by which the pattern remains unaltered ; and fifth, working in a smaller radius than any other cutter. Those who have used these cutters state that they do the work smoother than any other cutter they have used, and we can ourselves testify to the beauty of moldings shown to us as the work of this machine. The machine is constructed in a neat, tasteful, and substantial manner, and cannot fail to do good work. This machine, as we have described it, is covered by several patents, the property in which is now vested in the firm of Mellor Orum, 448 North Twelfth street, Philadelphia, Pa., who may be addressed for further information. TIic Hartford Steam Boiler Inspei-tlon and Insurance 4'ompauy. This Company makes the following report of inspections for the month of September: Visits of inspection made, 357 ; boilers examined, 541 ; external examinations, 471 ; internal examinations, 170 ; number tested by hydraulic pressure, 67. The whole number of defects discovered are 513, only 39 of these, however, are regarded as especially dangerous. These defects are as follows: Furnaces out of shape, 9; fractures in all, 313—5 dangerous. These fractures are usually the result of over-heating, and accumulation of deposit on the crown sheet. In one of the above cases, the tube sheet of an externally fired cylindrical boiler was in a very dangerous condition, and but for the timely examination of the inspector, might have been the cause of a serious accident. In other cases the fire sheets of boilers have been found covered with deposit, and, consequently, over-heated and badly fractured. These cases show the importance of frequently cleaning the fire sheets of boilers from all deposit and incrustation. Burned plates, 33—1 dangerous ; blistered plates, 35—6 , dangerous ; cases of incrustation, deposit, and scale, 74—4 dangerous ; external corrosion, 34—4 dangerous ; internal grooving, 7 ; water gages out of order, 30 ; blow-out apparatus out of order, 6 ; safety valves over-loaded, 31- dangerous (three of these last were corroded fast in their seats, showing that they had not been raised for some time, and being useless for the purpose intended) ; steam gages otit of order, varying from 7 to 30 pounds, 44—3 dangerous ; boilers without gages, 4 ; cases of deficiency of water, 4—3 dangerous ; faulty bracing, 11. Relative to the , mal-construction of boilers, we copy as follows from the report of L. E. Fletcher, Chief Engineer of the Manchester Steam User's Association. In commenting upon an explosion, he says : " The simple.cause of the explosion was the mal-construction of the boiler. The manhole was not strengthened as it should have been, with a substantial cast-iron mouth-piece, through the neglect of which so many explosions have from time to time occurred. At present boiler makers can palm off on the public bad boilers. and steam users employ them with the certainty that when they explode with fatal consequences they will, by the help of a coroner and his jury, be publicly absolved from all responsibility and the event proclaimed to be accidentah" One of our inspectors writes us that he has examined a set of boilers which have been used for upward of thirty years, and although the water is in some respects bad, he regards them as in first-class condition. They have been under the care of a competent engineer for twenty-five years. This shows what careful management will do in prolonging the working age of boilers.