THE Quarterly Journal of Science forJuly, 1877, contains an able and interesting article by James Croll, LL.D., F.R.S., on the age and origin of the sun's heat. The theory of Dr. Croll may be regarded as a compromise between the mathematicians, represented by Thomson, Tait, and Newcomb, and the geologists of the in uniformitarian school, represented by Playfair, Lyell, Darwin, etc. The principal points of this remarkable paper are : 1. That, as had been estimated by Sir William Thomson and others, but twenty million years' heat could beproduced by the falling together of the sun's mass. 2. That not less than five hundred millions of years have been required for the stratification of the earth's crust at the present rate of subaerial denudation, and hence that the gravitation theory of the origin of the sun's heat is incompatible with geological facts. 3. If we suppose two solid opaque bodies, each equal to half the sun' s mass, to fall together m consequence simply of their mutual attraction, the collision would instantly generate sufficient heat to reduce the entire mass to a state of vapor. If, m addition to the motion resulting from their mutual attraction, we suppose the bodies to have had an original or independent motion toward each other of 202 miles per second, the concussion would produce 50,000,000 years ' heat; a motion of 6,8 miles per second, together with that due to their mutual attraction, would generate 200,000,000 years ' heat; and a velocity of 1,700 miles per second would generate an amount of heat which would keep up the supply at the present rate for 800,000,000 years. 4. The sun and all visible stars may have derived their heat from the collision of cold, opaque masses thus moving in space. The nebulre are the products of the more recent impacts, and the stars have been formed by the condensation of ancient nebulre. 5. This theory, while accepting the doctrine of the conservation of energy, indicates at the same time a possible supply of heat for several hundred millions of years, thus satisfying all moderate demands for geological time. The mathematical correctness of the theory here stated will not be called in question. We shall consider merely the probability of the facts assumed as its basis. To the present writer the hypothesis seems unsatisfactory for the following reasons : 1. The existence of such sidereal bodies as the theory assumes is purely conjectural, unless it be claimed tat lost or missing stars have become non-luminous, of which we have no conclusive evidence. 2. But, granting their exi stence, we have no instances of stellar motion com parable with those demanded by the hypothesis, the velocity being II most cases less than 50 miles and in no case exceeding 200 miles per second. 3. If the two masses by whose collision the sun is supposed to have been formed were very unequal, as would be most probable, the amount of heat generated would be correspondingly less. 4. Such collisions as the theory assumes are wholly hypothetical. It is infinitely improbable that two cosmical bodies should move in the same straight line ; and of two moving in different lines, it is improbable that either should impinge against the other. Comets pass round the sun without collision against it. 5. But, granting these difficulties removed, let us suppose that about 800,000,000 years ago two cold, opaque bodies, each containing one-half the matter of the solar system, were approaching one another in the same straight line, each at the rate of 1,700 miles per second;* that on meeting, their motion was transformed into heat, and that their united mass was at once reduced to vapor, the great question yet remains, How much of the period represented by these 800,000,000 years' heat can be claimed as geological time? The nebula formed by the collision would extend far beyond the present orbit of Neptune. The amount of heat radiated in a given time from so vast a surface would doubtless be much greaterthan that now emitted in an equal period. No considerable contraction could occur until a large proportion of the heat produced bv the im pact had been dissi pated in s pace. It has been shown by Trowbridgef that, with a temperature at the sun's surface of twice its present intensity, the solar atmosphere would be expanded beyond the earth's orbit. The conclusion seems inevitable that much the greater part of the 800,000,000 years' heat must have been radiated into space before the planets were separated from the solar mass, and, consequently. that the amount of geological time cannot, to any great extent, have exceeded the limits indicated by the researches of Sir William Thomson. Upon the whole, it seems more difficult to grant the demands of Dr. Croll's hypothesis than to believe that in former ages the stratification of the earth's crust proceeded more rapidly than at present. The former, as we have seen, has no sufficient basis in the facts of observation. On the other hand, if our planet has cooled down from a state of igneous fluidity, the great heat of former times must doubtless have intensified both aqueous and atmospheric agencies in producing modifications of the earth's exterior. — American Journal of Science and Arts. THE working out of the results obtained by the transit of Venus expeditions sent out by the German government were expected to have been far enough advanced for publication in the year 1877, but it has been found that this task causes more difficulties and expense than had been at first anticipated, and a demand has been consequently made for an extra credit of £500 to defray the additional costs.—Engineer. SPONTANEOUS combustion of substances can only take place when they are so packed that small increments of heat may be stored up. If they are freely exposed to the air the heat evolving action which goes on with it may be increased rapidly; but its effect is dissipated. PHOTOGRAPHY by electric light is again occupying attention in London, and, it is said, successfully in portrait work, the electric light being thrown from a parabolic reflector through a Fresnel lens upon the sitter. The light is soft and uniform. * This is the greatest velocity mentioned by Dr. Croll. An increased rate of motion would, of course, produce more heat, but the hypothesis would be open to the same objections. t Proc. Am. Phil. Soc., vol. xvi., pp. and National Quarterly Review, March, 1877, p. 292.