104 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



earth consisting almost entirely of chalk. That chalk was 

 actually held in solution by fixed air was proved by mixing 

 lime-water with an excess of water saturated with the gas, 

 when a clear solution was produced, whereas the use of less 

 fixed air caused the lime to be precipitated in the form of 

 chalk. Cavendish writes : 



" Calcareous earths, in their natural state, i.e. saturated 

 with fixed air, are totally insoluble in water ; but the same 

 earths, entirely deprived of their fixed air, i.e. converted 

 into lime, are in some measure soluble in it ; for lime-water 

 is nothing more than a solution of a small quantity of lime 

 in water. It is very remarkable, therefore, that calcareous 

 earths should also be rendered soluble in water, by furnish- 

 ing them with more than their natural proportion of fixed 

 air, i.e. that they should be rendered soluble, both by 

 depriving them of their fixed air, and by furnishing them 

 with more than their natural quantity of it. Yet strange as 

 this may appear, the following experiments, I think, show 

 plainly it is the real case." 



" A bottle full of rain water was inverted into a vessel of 

 rain water, and some fixed air forced up into the bottle, at 

 different times, till the water had absorbed as much fixed air 

 as it could readily do ; 1 1 ounces of this water were mixed 

 with 6J of lime water. The mixture became turbid on first 

 mixing, but quickly recovered its transparency, on shaking, 

 and has remained so for upwards of a year." 



" Lest it should be supposed, that the reason why the 

 earth was not precipitated in the foregoing experiment, was, 

 that it was not furnished with a sufficient quantity of fixed 

 air, the following mixture was made, which contains the 

 same proportion of earth as the former, but a less proportion 

 of fixed air: 4! ounces of the above-mentioned water, 

 containing fixed air, were diluted with 6J of rain water, and 

 then mixed with 6| ounces of lime-water. A precipitate was 

 immediately made on mixing, which could not be re-dissolved 

 on shaking" (Phil. Trans., 1767, 57, 101 and 104-105). 



This observation was confirmed by Lavoisier, and also by 

 Bergman, who noticed that if 



