68 



BENJ. PIKE S, JR., DESCRIPTIVE CATALOGUE. 



stance, if you suspect some substance to be present in the 

 solution and wish to ascertain whether it is so or not, you 

 pour a small quantity of the solution into a test tube or glass, 

 and then add any other solution known to act on the sub- 

 stance you suspect to be present. The solution you add is 

 Fig. 502. known in chemistry as a re- 



agent. You must then judge 

 by the result you observe in the 

 solution, on the re-agent being 

 added, whether the substance 

 you suspect is present or not. 

 Price, 3 in. 3 Sets. doz. 

 " 4 in. 50cts. " 

 " 5 in. 63cts. " 

 " 6 in. 75cts. " 

 " 6 in mahogany 



frame, $1.00. 



Air Thermometer. (Figs. 504 and 505, next page.) This 

 instrument consists of a glass vessel or bottle, with a broad 

 base and narrow neck, containing a colored liquid, a ; along 

 tube having a glass bulb blown at one end, 6, the other ex- 

 tremity of the tube being open, and plunged in the colored 

 liquid ; and a scale of equal parts, c. Heat is applied to the 

 bulb to expel a part of the air, which permits a portion of 

 the colored fluid to rise in the tube ; this fluid column indi- 

 cates the slightest changes of temperature by rising when it 

 is diminished, and falling when it is increased ; because every 

 increase of temperature expands the air in the bulb, occa- 

 sioning it to press upon the fluid column and force it down, 

 while any decrease of temperature contracts the bulk of the 



