532 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



each containing a small tube filled with purified phosphoric 

 anhydride. The mixture was sealed in by a plug of fusible 

 metal and put aside to dry (Fig. 57). 



" Comparative tubes were made at the same time from 

 the same length of tubing and treated in precisely the same 

 way, except that no phosphoric oxide was sealed up in them. 



After 10 days' drying, two 

 such tubes were heated 

 side by side in the same 

 Bunsen burner flame. In 

 twelve experiments, the 

 wet tube exploded and 

 the dry tube did not. In 

 only one experiment has 

 a dry tube exploded, but 

 in this case the tube had 

 been carried for some 

 miles by hand, and most 

 probably some of the 

 phosphoric oxide had 

 been shaken into the part 

 of the tube which was 

 heated. In two experi- 

 ments, where only 2 days' 

 drying had been allowed, 



PyPJl'P.. OXIDE 

 FUSIBLE METAL 



water was slowly formed 

 in the dried tube, but 

 although visible moisture 

 was present, no explosive 

 took place, and a slow combination only 



FIG. 57. H. B. BAKER'S APPARATUS FOR 

 HEATING DRIED MIXTURES OF HYDRO- 

 GEN AND OXYGEN (i) by means of a 

 burner, (2) by a silver wire heated to fusion. 



combination 

 occurred." 



Silver was melted in the dried gas, and small sparks 

 were passed through it without producing an explosion : 

 but the melting of platinum and the passage of larger 

 sparks always detonated the mixture. 



Conditions under which chemical change takes place. 

 The experiments described in the preceding paragraphs 

 show that chemical changes frequently involve factors which 



