148 ELEMENTS OF LABORATORY WORK 



related to them, sometimes coinciding with them ; but the 

 theories by means of which they are derived gain no confirma- 

 tion from the process. The strongest reason which we have 

 come across in our past observations for supposing that atoms 

 have a real existence, and that the atomic masses represent 

 their relative masses, lies in the fact that the supposed relative 

 masses of atoms are those masses which are thermally equiva- 

 lent. On the other hand, chemical equivalents agree with 

 electric equivalents. The obvious suggestion is that changes 

 of temperature are changes in the atom as a whole changes, 

 in fact, in the motion of the atom while chemical and 

 electric changes are disturbances of a state of equilibrium 

 which many considerations urge us to regard as very com- 

 plicated. 



Additional Exercises and Questions. 



1. Heat small quantities of iodine, ammonium chloride, and lead 

 nitrate. Plug the vessels with cotton-wool. Compare the changes 

 taking place, and closely investigate the substances when they are 

 cold. 



2. Perform an experiment with a view to proving that chemical 

 change causes no alteration in the total amount of matter taking part 

 in the change. 



3. Analyse roughly, by using phosphorus, the air which has been 

 held in solution by rain-water, and state whether your results, when 

 compared with the analysis of ordinary air, would lead you to describe 

 air as a chemical compound or a mixture of gases. The air may be 

 expelled from the water and collected by filling a flask, and a tube in 

 connection with it, completely with the water. The tube is bent so as 

 to dip beneath the mouth of an inverted vessel standing over water and 

 containing water. The water may be renewed in the flask until sufficient 

 air is obtained. 



4. Place moist iron powder (not filings, which are generally oily) in 

 a confined volume of air over water, and leave for several days. The 

 larger the surface of powder exposed, the quicker will be the change. 



5. Substitute oxygen for air in the experiment in Section 89. 



6. Observe the action of acids and alkalies on litmus. Observe, by 

 the use of litmus, that definite quantities of acids are required to 

 4 neutralise ' definite quantities of alkalies. Use burettes. 



7. Prepare some solid sodium chloride from hydrogen chloride and 

 sodium hydrate. 



