ON ARTIFICIAL REFRIGERATION. 917 



32^ Falir., and to be again dissolved" in tlie central cliamber. To pro- 

 duce " intense degrees of cold in the apparatus," a smaU quantity of ice 

 or snow is put into the central chamber. 



In 1858, Mr. Siemens improved the construction of the refrigerator 

 in this machuie, and in the system of evaporating the spent solution for 

 the purpose of recrystallization or reproduction of the salt or compound 

 which has been dissolved. He used evaporating-pans over a fm-nace, 

 or the flues thereof, in such a manner as to aflbrd the means of drawing 

 off the contents of one or more pans into one or more other jpans. 



Another device for revolving ice-moulds with a freezing-mixture around 

 them was iiatented in England, in 1862, by Giovanni Battista Toselli, 

 and this form of apparatus is being sold in Paris. The invention consists, 

 first, in the vertical rotation of the liquids to be congealed ; secondly, 

 in the very simple shape of the machine with concentric sides and oppo- 

 site openings, whether such machine be made in whole or in i)art of 

 metal ; thirdly, in the said machines being suitable for the production 

 of ice by chemical means. 



M.— GASES AND THEIR LIQUEFACTION^. 



Yan Helmont introduced the word " gas," and in 1752 he established the 

 existence of gas sylvestre (carbonic acid), which Black, three years later, 

 termed fixed air. To Van Helmont is due the distinction between a gas 

 and a vapor. Aeriform fluids would not liquefy in cooling, whereas, 

 vapors, he said, required heat to maintain them in the fi'ee molecular or 

 gaseous state. 



Daniel Bernouilli first stated that gases are formed of material parti- 

 cles, free in space, and animated by very rapid rectilinear movements. 

 The tension of elastic fluids results from the shock of these particles 

 against the sides of the containing vessels. The gaseous molecules 

 manifest the energy of motion termed kinetic (from -/.r^iw^ I move). 

 Lucretius held that the different properties of matter depended on such 

 a motion. The law of Boyle or of Mariotte follows as a natural conse- 

 quence of this idea, and it is this law which interests specially ail those 

 who are engaged in the liquefaction of gases and the abstraction of heat 

 by these from surrounding objects, as they return to the gaseous state. 



As Professor Wurtz puts it in a recent lecture : * '' Suppose a gas occu- 

 pying a certain volume, and composed of a definite number of material 

 particles, or molecules, so called, to be contained in a closed vessel, such 

 as the c^dinder of an air-pump, the pressure on the piston will be deter- 

 mined by the number of shocks of the molecules diflused through the 

 neighboring stratum of gas. If, then, the volume of gas be reduced, the 

 number of particles in this layer will be increased as well as the sum of 

 the shocks, and the pressure will be increased in proportion thereto. 

 Temperature is determined by these movements of gaseous molecules. 



* On the constitution of matter in the gaseous state, being the Faraday lecture deliv- 

 ered Noveniher 12, 1878, at the Eoyal Institution, Loudon. 



