CARBON AND SOME OF ITS COMPOUNDS 175 



Changes in mass when acids act upon chalk. If a certain mass 

 of chalk or limestone is weighed, and an acid is then allowed to act 

 upon it until no more gas is given off, it is found that the chalk always 

 loses mass in a given proportion. When TOO grams of chalk are treated 

 with enough acid to expel all the gas which puts out a flame, the loss 

 in mass is always 44 grams. Now, as none of this gas is given off when 

 acids come into contact with quicklime (the substance which is left 

 when chalk is strongly heated), the conclusion which must be come to 

 is that quicklime contains none of it, and that strong heat acts in the 

 same way as the acid in expelling the gas. The loss of 44 grams in 

 mass therefore shows that 100 grams of chalk contain 44 grams of 

 the gas which puts out flames, and called carl>on dioxide ; and the 

 experiment of strongly heating chalk proves that the remaining 56 

 grams are quicklime, or that 



100 grams are 56 grams 44 grams 



of made up of and of 



CHALK of QUICKLIME CARBON DIOXIDE. 



Action of acids and of heat on chalk compared. These results 

 are of such great importance that it is worth while to look at them in 

 another way to try to understand them thoroughly. It has been 

 learnt that 1 JO grams of chalk when strongly heated lose in mass and 

 give 56 grams of quicklime. Also, when 100 grams of chalk are acted 

 upon by acids there is a loss of mass too, and 44 grams of an invisible 

 gas, which puts out flames, are given off, and this gas turns blue litmus 

 red. 



But by adding 56 and 44 together the result is exactly 100, and it 

 looks as if the quicklime and carbon dioxide together make up chalk. 

 As chalk can actually he made from the combination of these two 

 compounds, there need be no room for doubt, and it may be written 

 as a fact, that 



CHALK made QUICKLIME and 



Up of 'IOX1DE. 



QUESTIONS ON CHAPTER XXVII. 



I :;.">. Describe how you would prepare carbonic acid gas by two different 

 methods. In what respects does this gas differ from nitrogen ? 



13f. (livo a brief summary of the conclusions arrived at from your study 

 of chalk. 



' 'h.'ilk when strongly heated affords two constituents ; state clearly 

 UK- projM-rtirs of tlif-se constituents; do you regard them as elements? 

 if not, state the evidence in favour of their being compound substances. 



