K. MECHANICS AND ENGINEERING. 399 



not only at an elevated temperature, but precipitates and neu- 

 tralizes in cold water all these foreign bodies. For this rea- 

 son the water may be made to undergo a preliminary purifi- 

 cation before passing into the boiler. Our space will not 

 allow us to give the details of the chemical theory by which 

 the different substances referred to play their parts in pre- 

 venting or removing the deposits in question, although fully 

 set forth in the original communication of Mr. Weiss. We 

 may state, however, that two pounds of the preparation are 

 said to be sufficient for eighteen hundred quarts of water. 

 1 B.July 4,16. 



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PREVENTING THE SPONTANEOUS COMBUSTION OF COAL. 



With the view of preventing, as much as possible, the 

 chances of spontaneous combustion of coal on board ships, 

 and in coal-bunkers on steam vessels, Dr. Lachmann recom- 

 mends that coal as free as possible from iron pyrites be se- 

 lected, and that the coal be loaded rapidly and in as dry a 

 state as practicable. It should be stowed on board not too 

 closely, and, as far as may be, all access of air and water is to 

 be prevented. A sprinkling of coal-tar will, it is said, add to 

 the security against spontaneous ignition. 1 A 7 JSFov. 4, 225. 



WEATHERING OF COAL. 



An important communication has been made by Dr. Rich- 

 ters to a German journal upon the influence of atmospheric 

 agencies on stone coal exposed to the air in coal-yards and 

 other localities. In this memoir he states that the property 

 which coal has of taking up oxygen when heated gently (as 

 to 375 Fahr.) is modified essentially by its percentage of dis- 

 posable hydrogen. This first of all becomes oxidized, togeth- 

 er with a certain portion of the carbon, since, on the one hand, 

 water is formed, and, on the other hand, the oxygen enters 

 directly into combination with the coal. Also, that the car- 

 bon of stone coal possesses, at a temperature of about 375 

 Fahr., a variable affinity to oxygen, as the smaller portion 

 (five or six per cent, of the total amount) combines with it 

 and forms carbonic acid, while the rest, at the given tempera- 

 ture, shows little or no affinity for oxygen. While these two 

 propositions respecting the oxidation of coal when heated 

 can be established, our author adduces experiments to show 



