15 



These numbers differ too much to warrant us in accepting the for- 

 mula proposed by M. Terreil as the true one. 



M. Terreil states, that even at 200° cent, hydrocarbonate of zinc loses 

 only hygroscopic water; this statement appears singular, especially when 

 we recollect the interesting results of M. Damour,^' who found that even 

 the zeolites, with the exception of analcime, possess the property of 

 losing considerable quantities, and sometimes even the whole of their 

 hydrated water, either when placed in a perfectly dry atmosphere, or 

 when exposed to temperatures comprised between 40° cent., and in- 

 cipient redness, and of again taking it up. The loss of water which 

 hydrates sustain when heated, depends not only upon the temperature 

 to which they are exposed, but likewise upon the relative facility with 

 which the air in contact with them is changed, and upon the duration 

 of the exposure. In order to test this point, the percentage of water 

 and carbonic acid in a piece of perfectly white compact hydrocarbonate 

 was determined by the loss which it sustained by ignition, in its air- 

 dried state, after an hour's exposure to a temperature of 130° cent, in 

 an oil-bath, and after an exposure of five or six hours to a tempera- 

 ture ranging between 150° to 180° cent., and with frequent exposure 

 to the air. A similar experiment was tried with a fragment of pure 

 white friable bergmehl-like hydrocarbonate. The following table con- 

 tains the results of these experiments : — 



Compact Friable light 



Mineral. Mineral. 

 Total HO, and CO2 in air-dried mineral, 25-738 . . 28-380 



Loss in one hour at 130°, 2-041 . . 3-151 



Lossinsixhoursat 150°to 180°, . . 14-423 . . 18-571 



The following table represents the relative composition at each 

 stage : — 



Compact Mineral. Friable light Mineral. 



, A , , A 



AV nri-ori Dried at Dried at 150" Air- Dried at Dried at 150« 



Air-jjriea. ^^^^ jgQ„_ Dried. 130". to 180". 



ZnO,. . 74-262 . . 75-809 . . 88-898 — 71-620 . . 76-121 . . 92-302 



£9?' \ . 25-738 . . 24-191 . 11-102 — 28-380 . . 23-879 . . 7-689 

 HU, ) 



100-000 100-000 100-000 100-000 100-000 100-000 



These experiments show that not only does hydrocarbonate of zinc 

 lose hydrated water at temperatures under 200°, but even a considerable 

 quantity of carbonic acid. It is even probable, that in a current of 

 hot air at a temperature of 180° cent., it would be fuUy decomposed. 

 It may, however, be safely dried at the temperature of boiling water, or 

 even as high as 120° cent., provided it be not too long exposed to the 

 • heat. 



With the view of determining whether the composition of the hy- 

 drocarbonate is always constant, a large number of specimens, exhibiting 



* Compt. rend. t. xllv. p. 975. 



