L. TECHNOLOGY. 531 



acquire a concavity in the middle of three fourths of an inch. 

 If air be blown into the cavity, on the other hand, the plate 

 becomes convex. It is expected that the process can be made 

 so perfect as to render the convexity uniform for two plates, 

 which, when cemented around by their edges, and filled with 

 some strongly refracting liquid, will serve the purpose of a 

 cheap and powerful lens. Indeed, an inventor in Baltimore 

 lias realized this expectation, and succeeded in producing 

 lenses of great power and unusual cheapness. 6 C\ Septem- 

 ber 28,1871,389. 



AMMONIA ENGINES. 



The Abbe Moigno claims for France the discovery of the 

 applicability of ammoniacal gas as a motive power, and cites 

 a communication of Tellier, the well-known inventor of the 

 ice machine, to the Academy of Sciences at Paris, made some 

 time ago. In this article it is stated that the availability of 

 ammonia for the purpose consists, first, in its great solubility 

 in water ; second, in its ready liquefaction ; third, in the facul- 

 ty which it possesses of furnishing industrial pressure at the 

 ordinary temperature ; fourth, in the possibility of superheat- 

 ing its vapor without reaching too high a temperature ; and, 

 fifth, in the possibility of collecting the vapors expended by 

 their solution in water, and then recovering them again, to be 

 used anew in the operation. The more important applica- 

 tions of this gas, he thinks, will be in railroad traveling, for 

 the purpose of working high grades, and as a motive power 

 in tunnels, where smoke and burned air would not be desira- 

 ble ; also in mines, and in the minor industries, where a cheap 

 and safe motive power is needed. 3 B, Sept. 14, 1871, 525. 



TINNING METALS. 



A bath can be prepared for coating any metallic substances 

 with pure tin by dissolving ordinary tin in hydrochloric acid 

 and precipitating the pure metal by means of an alkaline lye. 

 The deposit is to be washed, and then introduced into a solu- 

 tion of cyanide of potassium or caustic potash. When com- 

 pletely dissolved, some hydrate of lime is to be added to the 

 solution. Into this bath plates of tin and the articles to be 

 coated are to be immersed, and the two connected in the or- 

 dinary manner. 13 C, July 1, 1871, 58. 



