To measure the latent heat of fusion two meth ods have been used successfully; one is an ob vious application of the method of mixtures: the other is to secure the fusion by means of the heating action of an electric current, whose in tensity and electromotive force may be measured. thus giving the quantity of energy consumed in producing the fusion.
To measure the latent heat of evaporation, two similar methods have been used, and a third also. If the vapor is conducted through a long spiral tube surrounded by water. the vapor will con dense, the temperature of the water will be raised. and thus we have simply the method of mixtures. Or a liquid may he made to evaporate by means of a reduced pressure, and the conse quent fall of temperature may he balanced by the heating action of a known electric current. Again, if a liquid inclosed in a Bunsen calorim eter is caused to evaporate, the surrounding water will he frozen—just the reverse of the gen eral use of the apparatus—and the quantity frozen may be measured. For full description of these various methods for measuring latent heats of all kinds, reference should he made to general treatises on heat. Preston. Theory of (Lon
don, lS94), is perhaps the best book of reference.
Another class of calorimeters is designed to enable the observer to ascertain the number of calories furnished when certain fuels, such as coals, oils, etc., are allowed to burn under definite conditions. In these the coal is placed in a hol low steel cylinder which can be tightly closed, oxygen is admitted tinder high pressure, and com bustion is started by means of an electric cur rent through a fuse wire. The cylinder is kept immersed in water, the rise of temperature of which is observed, and the energy generated thus measured. Calorimeters constructed on a large scale are used to measure the amount of heat given off by an animal or human being, the amount of food and air supplied to the subject of the test being recorded. Prof. W. 0. Atwater, of Wesleyan University, has carried on a num ber of experiments with such an instrument. and has ascertained the fuel value of various foods. His results are to be found in a series of bulle tins issued by the United States Department of Agrienit ure.