630 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



trated by two similar series of air- waves; producing separately 

 two sounds; but when brought in opposition to each other, the}-- 

 are completely neutralized, silence being the result. 



In every photographic process used thus far, the action of silver 

 is the sine qua non, although a very small proportion of the whole 

 amount employed becomes a part of the picture. Hence it is an 

 interesting problem to determine, by the use of other salts, the 

 least quantity of silver required to produce the desired result. 



White Lead and Saltpeter. 



Clarence Delafield, of Staten Island, has patented a process for 

 manufacturing carbonate of lead. He claims his product is whiter 

 and purer than can be produced by the old Dutch process. It 

 consists in mixing a hot solution of nitrate of lead with a hot solu- 

 tion of carbonate of potash, and passing into the mixture super- 

 heated steam, which, it is claimed, prevents the formation of 

 hydrated oxyd of lead, and by his peculiar manipulation the 

 whole is changed to carbonate of lead. After the salts of lead 

 have been precipitated, the remaining solution of nitrate of potash 

 is drawn off and evaporated, thus leaving very pure crystals of 

 saltpeter. 



On Blue Slags. 



C. Mene, in a paper read before the French Academy, stated 

 that the presence of titanic acid is not always the cause of the 

 blue color in slags, as is generally supposed. From nineteen 

 analyses he concluded this acid was the coloring matter in vitreous 

 but not in compact slag. M. Cheureul remarked, after the read- 

 ino- of this paper, that the blue coloration might sometimes be 

 due to the presence of particular oxyd of iron. 



Deterioration of Bituminous Coal. 

 Herr Grundmann, of Farnowitz, in Germany, and Herr Varen- 

 trapp, have recently made experiments which prove conclusively 

 that soft coal — both that used for cooking and for gas — is slowly 

 oxydized at ordinary temperatures. The conclusions of the latter 

 are briefly stated by Prof. Rockwell, in The American Journal of 

 Mining, as follows: That during the period of exposure the coal 

 underwent a process of slow combustion, taking up oxygen and 

 giving off the volatile products of oxydation. In this decomposi- 

 tion, air and moisture play the principal part, and warmth is the 

 condition of promoting it. The degree of heat determines the 



