BY DR. LADDER BRUNTON. 569 



The Hydrometer. The hydrometer is an elongated glass bulb which 

 is weighed at one end so as to make it float upright, and is prolonged at 

 the other end into a stem, graduated in such a manner that the number 

 of the division up to which the instrument sinks expresses the specific 

 gravity of the liquid in which it is placed. As every instrument reads 

 accurately only at the temperature for which it is constructed, the 

 liquid must be brought to the proper temperature before the instrument 

 is used. In using the hydrometer, the liquid must be placed in a cylin- 

 drical glass vessel, deep enough and wide enough to allow the instru- 

 ment to float freely in it without coming in contact with the sides or 

 bottom. The froth, if any, is then to be removed from the surface with 

 a piece of blotting-paper, and the hydrometer allowed gently to sink 

 into the liquid. The mark on the scale, which coincides with its sur- 

 face, indicates the specific gravity. To read this correctly, the eye 

 must be brought to a level with the surface of the liquid. When this 

 is the case, the surface presents the form of a meniscus, assuming the 

 aspect of an ellipse when the eye is either raised or lowered. To insure 

 accuracy, the reading should be repeated once or twice, the hydrometer 

 being down in the liquid between each two observations. 



Specific Gravity of Solids. The specific gravity of a solid mass, the 

 substance of which is insoluble, is ascertained by weighing it first in air 

 and then in water. The difference between these weights is equal to 

 the weight of its own bulk of the water which it displaces. The specific 

 gravity is therefore got by dividing the weight of the solid in air by the 

 difference between its weight in air and water. The weight of solids 

 may also be ascertained by immersing them in fluids of known density 

 till they float. Thus the best way of ascertaining the specific gravity of 

 the substance of the brain, or any other organ, is to prepare a graduated 

 series of solutions of common salt of different densities, and to immerse 

 the solid, first in one, and then in another, till a solution is found in 

 which it floats indifferently at any height. 



217. Volumetrical Analysis. For volumetrical analyses, measuring 

 flasks, measuring glasses, pipettes, burettes, and other accessory appa- 

 ratus are required. 



Mi'.*uring Flasks. These flasks, of the form shown in fig. 346, are 

 used for dissolving substances for the preparation of standard solutions, 

 etc. They should have tolerably wide mouths, and be furnished with 

 well-fitting stoppers, so that they may be shaken without risk of loss. 

 The graduation mark should be ju:t below the middle of the neck. 

 Flasks are used of capacities varying from 100 centimetres to a litre. 

 Gr<><] netted cylinders, such as that shown in fig. 347, generally called 

 test-mixers, are used for the same purpose. 



Pipettes. A pipette is a glass tube of the shape shown in fig. 348, and 

 when filled up to the mark on the neck it should deliver the exact quan- 

 tity of fluid which is marked upon it. Some pipettes are graduated so 

 as to let the exact quantity run out by its own weight ; others, to de- 

 liver the right amount only when the liquid is blown forcibly out. The 

 former arc to be preferred. Another kind of pipette is graduated along 

 the greater part of its length, so as to deliver different quantities at will, 

 but it is not so accurate as the others. In using pipettes, the liquid to 

 be measured is to be put into a test-glass or small beaker ; the lower 

 end of the pipette is then immersed in the liquid, which is to be sucked 

 up till it stands somewhat above the mark on the neck of the pipette. 

 The upper end of the pipette must then be quickly covered with the 

 moistened tip of the forefinger, so as to prevent the liquid from flowing 

 out. The mark on the neck is next brought to a level with the eye. and 

 the tip of the finger gently raised so as to allow the liquid to escape slowly 

 till it stands opposite the mark. It is then allowed to run out into a clean 



