51)0 Professor Sir James Dcwar [March 25, 



ill which they exist in water. Two insulated wires are let into the 

 bulb, by means of which, on attaching them to an induction coil, we 

 can explode the gases by the electric spark. The instant the explosion 

 takes place, a corresponding vaporisation of liquid air follows, and 

 from its amount as shown in the receiver we can determine the 

 heat of combination of oxygen and hydrogen. In a similar manner 

 the heat of the electric spark itself can be determined. Two fine 

 wires, each passing through a fine glass tube, are passed down into 

 the calorimeter until their ends are below the liquid surface, or they 

 may be connected to a small sparking tube immersed in the liquid 

 air. On attaching them to the induction coil and making contact, 

 a spark passes in the liquid, or in the glass bulb, which immediately 

 evaporates some of it, and from its gas volume in the receiver we 

 find the heat generated by the spark. 



The determination of specific heats and latents is a matter of gi'eat 

 importance. Latent heats of evaporation are usually determined 

 under the ordinary atmospheric pressure : alteration of pressure has,, 

 in general, but a small effect on the latent heat of solidification. 

 Specific heats, on the other hand, vary with the temperature, so that 

 it becomes important to measure tliem at various temperatures. This 

 is usually done by observing the mean specific heats over finite ranges 

 of temperature, as, for example, between the boiling point of water 

 and its freezing point, between the freezing point of water and the 

 boiling point of carbonic acid, between this latter point and the boil- 

 ing point of oxygen, and between this and the boiling point of 

 hydrogen ; or, we may go upwards, say, from the boiling point of 

 water to the melting point of paraffin, then to the boiling point of 

 sulphur, and so on. As so many properties of substances tend either 

 to a maximum or a minimum, as we approach the absolute zero, there 

 is great interest in examining the values of the specific heats of 

 substances at low temperatures. Dulong and Petit discovered a 

 simple, and very general law, according to which the product of the 

 specific heat of a substance, in the solid state, and its atomic weight 

 is constant. For example, let us take two small pieces of metal of 

 equal weight — one, lead, with the high atomic weight of 207 ; the 

 other, aluminium, whose atomic weight is only 27. On comparing 

 the evolution of gas when the lead is put into the calorimeter, with 

 that of the aluminium under the same circumstances, the very marked 

 excess of the evaporation with aluminium over that with lead is at 

 once apparent, and even a rough measure of the relative amounts of 

 gas evolved shows one to be almost seven times that of the other, a 

 ratio very nearly equal to that of their inverse atomic weights. 



Two of the most interesting substances to study in this connection 

 are carbon, in its two forms, diamond and graphite, and ice. 



A large series of observations were made upon carbon in both it^^ 

 forms, the ranges of temperature being from about 18° C. to the 

 boiling point of carbonic acid, thence to the boiling point of oxygen, 



