8 STUDIES IN PHYSIOLOGY 



process the carbon takes to itself some of the oxygen about 

 it and forms a colorless, tasteless, odorless gas which we 

 may call oxid of carbon. The more common names applied 

 to it are carbonic acid gas and carbon dioxid, the latter being 

 the preferable term. 



Test for Carbon Dioxid. In spite of the fact that carbon 

 dioxid has no color, taste, or odor, we can easily demonstrate 

 its presence in the following way. If we put some clear 

 limewater 1 into a bottle in which carbon dioxid has been 

 formed by burning wood, we find after shaking that the 

 liquid loses its transparency and becomes milky. This 

 change in limewater is always caused by carbon dioxid 

 and by nothing else. 



Mineral Substances. In our experiments thus far we 

 have watched the formation of water (oxid of hydrogen) 

 and the oxids of phosphorus, sulphur, and carbon. Our 

 match is at last reduced to ashes. These we know will not 

 burn. We may heat them to a red or even white heat, but 

 in most cases they remain wholly unchanged, and on cooling 

 resume their gray or white appearance. The ashes of the 

 wood are the mineral matters that were obtained from the 

 earth by the living plant. 



Definitions. If we keep in mind the experiments we have 

 described, certain important definitions will now become 

 clear to us. All the materials of which the universe is 

 composed may be divided into two classes, namely, chemical 

 elements and chemical compounds. 



Elements. Of the materials of the universe there are 

 certain substances which chemists are unable to make any 

 simpler in their composition. Hence they are called chemi- 

 cal elements. Over seventy of these elements are known, 

 some of the most common being the carbon, oxygen, and 

 sulphur that we have been studying. The names of these 



1 Limewater is made by dissolving ordinary quicklime in water. 

 The solution, after it is strained or filtered through porous paper, is as 

 clear as ordinary water. 



