20 SECTION- PHYSICS. 



some of the instruments in the Exhibition, intended to facilitate the 

 ^lustration of this class of phenomena, and to enable the conclusions 

 of the undulatory theory of light to be compared with the results of 

 observation. He further described the method of adjusting the optical 

 bench, as arranged by himself, for repeating accurately the investi- 

 gations of Fresnel, Young, and others, and mentioned some results 

 obtained with this instrument in illustration of the close agreement 

 between observation and the deductions from the undulatory theory. 

 He also drew attention to the assistance which photography may 

 render in the study of interference phenomena, and exhibited some 

 photographs in illustration of his remarks. 



The PRESIDENT : I will now ask M. Pictet to give an account of 

 the Sulphurous Acid Ice-Machine upon R. Pictct's Anhydrous 

 System. 



M. RAOUL PICTET : In order clearly to understand the working of ice- 

 making machines, it is first of all necessary to explain the theory upon 

 which they are founded. Cold is produced by the evaporation of a volatile 

 liquid ; all liquids without a single exception in passing from a liquid 

 to a gaseous state, absorb a considerable quantity of heat, which is con- 

 sumed by the molecular work ; the constituent particles of the liquid 

 are strongly drawn together by the attraction of cohesion, heat 

 separates them, and thus does a great work. If, on the other hand, 

 vapours be compressed in a receiver, these vapours will transform 

 themselves into liquid, and will give out, during the process of conden- 

 sation, the same quantity of heat as that absorbed in their first change 

 of state. Thus, in theory, all liquids indiscriminately can, by their 

 passage through liquid or gaseous forms, be used for the artificial pro- 

 duction of ice. It is merely necessary to cause a volatile liquid to boil 

 in a closed vessel, surrounded by the water which one wishes to freeze ; 

 a pump continually draws away the vapours which are formed and com- 

 presses them in a condenser, in which they are condensed by means of 

 the temperature of a stream of water, and of pressure. 



The mechanical theory of heat allows the exact relation to be estab- 



