For all that has been said by us on the subject of heat and its effects, there is still a great number who have no correct information to guide them, and to whom some standard knowledge will be a benefit. This opinion we have formed from the many letters we have received, relating to questions connected with this subject—a subject, indeed, which possesses a universal interest because it encircles every practical art in the world, and is only circumscribed in its relations by the universe itself. Man with all his powers oi mind and great experience of memory treasured up in the records of men during many centuries, is still a very ignorant creature. He knows absolutely nothing of natural causes. When he sees an apple fall to the ground, or a stone flung from a sling return to earth again, he says, " the cause of this is the attraction of gravitation." Truly spoken, but "what is gravitation?" Aye, who can tell us that. Well, it is just the same with heat ; we know its effects, but what it is in itself, is wrapt in as much obscurity to our minds as what gravity is. We talk of heat and we talk of cold, but when we ask, "what is heat," we are answered, " it is the absence of cold, and when we ask, " what is cold,'' we are answered, 'HyjfeB absence ol heat."— Heat and cold 's jHSpi e middle links of which only haviyMBBjfrndered visible to us. Extreme cold ajMBlle human system in many ways lilHHnSrltne heat, but at the same time, they are generally opposites in their effects; great heat attenuates solids and renders them gaseous ; while great cold will condense and solidify these. We think it is extremely cold when the thermometer falls to 32 Fah. below zero, and it is dangerous to walk abroad when the atmosphere is at that temperature : but artificial cold has been produced as low as 220 by Natterer, and 166 by Faraday. The greatest natural cold ever measured was 56 below zero. Ice melts at 32, and this is called " the freezing point." A hot wind in upper Egypt has been found to be 117 or 19 above the temperature of human blood, consequently no man could live long in such an atrflcwphere. Alcohol boils at 173, water at 212. Tin melts at 442, lead at 590, and mercury at 662 ; 980 is a red heat, and 1141 according to Daniell, is the heat of a common rire ; brass melts at 1869 ; silver at 2283, and cast-iron at 3479. These temperatures to produce the effects stated, have been established by good authority, and those of our readers who are not acquainted with them should treasure this up for present or future use. SPECIFIC HEAT.—Equal bulks of different substances, such as water and mercury, require the addition of different quantities of heat to produce the same change in their temperature. It two similar glass bulbs like thermometers, the one containing water and the other mercury, be immersed at the same time in a hot water bath, it will be found that the mercury bulb is heated up to the temperature of the water bath in half the time the water bulb requires to be raised to the same temperature ; when exposed to the air, the mercury cools twice as fast. These effects must arise from mercury absorbing only half the heat that water absorbs in being raised to the same temperature. If we mix equal measures of water at 70 and 130, the temperature of the whole will be 100, but if we substitute for the water at 130 an equal measure of mercury, on mixing it with the water at 70, the temperature of the I whole will not be 100, but about 90 ; the mercury loses 40 of heat, which only raises the water 20. Hot mercury therefore possesses only one-half the quantity of heat that a like quantity of water does at the same temperature ; this fact is expressed in scientific language by saying of bodies, their capacity for heat. It is more convenient to express the capacities of different bodies for heat with reference to equal weights than equal measures. By experiment it has been found that a ? pound of water absorbs thirty times more heat than a pound of mercury in being heated to the same number of degrees ; the capacity I of water for heat, therefore, is thirty times D greater than that oi mercury. The capaci-, ties of these two bodies are in the relation of 1000 to 33, and it is convenient to express the capacities of heat for all bodies in relation to that of water as 1000—such numbers are the specific heats of bodies. The specific heat ot water is 1000, ice, 513, iron 113 79, zinc 95'55, mercury is really 3332, lead 3184. In equal weights of air and water, allowirg water to be 1, the capacity of air is 02669—in other words a pound of water has3'74 times the capacity for heat which air has, steam is to 1 of water,'8470'; carbonic acid is '2124, a lower capacity than air. The capacity of both ice and steam for heat, is less than an equal weight of water. The specific heat of a body, therefore, may change with its physical state. Air contains latent heat, for if it is condensed into one-fifth of its volume by a piston in a small cylinder, as much heat is evolved as will ignite some inflammable substances. As carbon acid gas has a.lower capacity for heat than air, it would be cheaper to use—taking fuel into consideration—than hot air for moving machinery. But as mercury is easily rendered volatile, and as its capacity for heat is 33 times less than water, it would certainly save fuel to employ it as a substitute for steam. This is no new idea, it has already been tried. About 15 years ago a beautiful little boat was built somewhere about the west of England, with an engine propelled by mercury, which was converted into gas by heat and used like steam, and was then condensed by some arrangement and used over and over again. It made one trip to Liverpool, and when it arrived at Princes Dock, the whole crew had to be sent to the hospital in a dreadful state of salivation, and that was the last of the mercury boat.