210 HOW TO WORK 



Secondly, there is no danger of the reagent becoming spoilt by 

 the introduction of various substances from without. If an ordinary 

 stoppered bottle be used, a drop of the fluid must be removed with 

 a pipette or stirring-rod, but if these should not be quite clean, 

 foreign substances may be introduced, and the reagent spoilt for 

 further operations. Carelessness upon this head will lead to the 

 greatest inconvenience, and give rise to serious mistakes. 



Thirdly, testing by means of these little bottles can be conducted 

 in a very short space of time, and they possess the advantage of being 

 packed in small compass, pi. XLV, figs. 279, 284. 



308. Capillary Tubes with India-rubber tied over the Top. 

 Dr. Lawrence Smith recommends that the tests should be kept in 

 bottles of two ounce capacity, and instead of a stopper, he inserts a 

 tube in the form of a pipette, the upper open end of which is 

 covered with a piece of vulcanised India-rubber, fig. 283. By 

 pressing this while the lower end is beneath the fluid, a portion of 

 the air is of course driven out, and a little fluid rushes in to supply 

 its place as soon as the pressure is removed. The tube with the 

 contained test solution may then be removed from the bottle, and by 

 again pressing the India-rubber, a drop, or a portion of a drop, is 

 very readily expelled. 



309. Testing for Carbonate and Phosphate of Lime, Phosphate 

 of Ammonia and Magnesia, Sulphates and Chlorides. Suppose the 

 nature of the substances composing certain forms of earthy matter 

 is to be ascertained. A small portion, about the size of a pin's 

 head, is placed upon the slide, and covered lightly with a piece of 

 thin glass. Next, a drop of nitric acid is placed near to the thin 

 glass. The acid soon reaches the sediment, and the disengagement 

 of a few bubbles of gas may be observed. These are, as it were, 

 temporarily pent up by the thin glass. If there should be any 

 doubt about the action of the acid, we may resort to examination in 

 the microscope, when, if there be very few bubbles, they may be 

 detected. The formation of bubbles of gas indicates the presence of 

 a carbonate. 



The acid solution may be neutralised with ammonia, when a faint 

 flocculent precipitate may be produced. After this has stood still for 

 a few minutes it should be covered with thin glass and examined under 

 the microscope. It may consist of amorphous granules and small 

 crystals, which, if allowed to stand long enough, will take the form of 

 triangular or quadrangular prisms (phosphate of ammonia and 

 magnesia, phosphate of lime). 



If we wish to ascertain the presence of sulphates, a little of the 

 nitric acid solution is treated with nitrate of baryt:s. An amorphous 



