Processes in the Useful Arts. 169 



raospheres. This liquid gas is contained in two cylinders placed at the two 

 extremities of the apparatus, and put in communication. In order to de- 

 stroy the equilibrium, it is sufficient to vary the temperature of the liquid 

 contained in one of the condensers. But the influence of the heat upon 

 the liquid gas is such, that for an elevation of 180° we obtain a pressure of 

 90 atmospheres, an enormous power, which, having no resistance but that 

 of the gas in the other condenser, tends to displace a piston with a force of 

 90 — 30 = 60 atmospheres. 



Mr Brunei has already constructed a working model of this engine, and 

 he is now occupied with a machine having the power of eight horses. 



The enormous heat necessary in the high pressure engines of Mr Per- 

 kins are not requisite in the present machine. It is indeed the peculiar 

 advantage of it, that it is not necessary to raise the temperature of the con- 

 denser above that of boiling water, in order to produce a pressure of 60 at- 

 mospheres. M. Thenard is of opinion that the great difficulty will con- 

 sist in obtaining a pressure of 30 atmospheres to condense the gas. When 

 this pressure is once obtained, nothing can be simpler than the play of the 

 machine, in which there will not be lost a drop of the liquid carbonic 

 acid. * 



3. Account of the Process of MM. Thenard and Darcet for Preserving 

 Substances from Humidity. 



On the 27th February 1824, there was read at the Academy of Sciences 

 of Paris, a Memoir by MM. Thenard and Darcet, on the employment of 

 fatty bodies for making coverings and unalterable plasters, and for making 

 moist places salubrious. This process, the effects of which have been esta- 

 blished by several years experience, consists in causing a mixture of one 

 part of oil and two parts of resin to penetrate, by means of an intense heat, 

 either porous stones or plaster. The bodies penetrated with this mixture 

 acquire afterwards a singular degree of solidity, and become absolutely im- 

 permeable to moisture. 



This process may be employed for rendering low and damp places salu- 

 brious. It was tried at the Sorbonne, and the expence of it was only 16 

 sous per square metre, or a square whose side is 39 English inches. The 

 other objects to which it is proposed to apply it are houses, statues placed 

 in the open air, has reliefs and sculptures in plaster, the ceilings and walls 

 of rooms intended for Fresco paintings, basins for holding water, and re- 

 servoirs for holding grain. 



M. Thenard exhibited to the Academy several objects of art executed in 

 plaster by his process. In order to show its efficacy, he exposed to the 

 open air for several years a bas relief, half of which was formed of ordina- 

 ry plaster, whilst the other half was prepared. This last hulf was perfect- 

 ly preserved, while the other displayed visible traces of disintegration. 

 This process does not resemble those which consist in covering bodies with 

 a sort of skin which keeps off humidity. The body is actually penetrated 

 with the mixture to the depth sometimes of several inches. 



" See an interesting notice of this invention in Le Globe, Tom. iii. No. 29, 

 28th Feb. 1826. 



