18 



MR J. Y. BUCHANAN ON THE 



M 



m 



- 5 



University Laboratory in Edinburgh, under Professor Crum Brown, in the years 

 1869 to 1872. A description of it was published in the Berichte der Deutsclien 

 Chemische Gesellscha/t (1871), iv. 338. Fig. 1 is a sketch of it. The stem of 



this instrument was made of glass tube having an external 

 diameter of about 1 centimetre, and a truly circular section of 

 uniform diameter. A slip of paper is attached inside the stem. 

 It is graduated on any convenient scale of equal lengths, and 

 the divisions are numbered upwards and downwards from the 

 zero point in the middle. The numerals from upwards have 

 the positive sign, and those running from the downwards have 

 the negative sign. 



The hydrometer is ballasted with mercury or shot, so that, 

 in its completed state, it sinks in the liquid used, at the atmo- 

 spheric temperature, exactly to the zero division in the middle 

 of the stem. 



The lower extremity of the instrument takes the form shown 

 in the figure, terminating in a hook K. The upper extremity 

 of the stem is closed with a cork, to which a suitable disc of 

 cardboard, M, is attached by sealing-wax. 



The hydrometer was originally constructed in order to illus- 

 trate the determination of the specific gravity of solid bodies. 

 The liquid in which it is to be immersed may be distilled water, 

 but other liquids, for instance sea-water, may also be used. 

 It is contained in a suitable cylinder, and should have the 

 temperature of the room in which the experiment is being 

 made. 



When it is proposed to exhibit the determination of the 

 specific gravity of any particular solid body, the hydrometer 

 is immersed in the liquid, in which it sinks until the zero 

 division on the scale is exactly in the plane of the surface of 

 the liquid. A suitable fragment or piece of the solid body 

 is then placed on the platform M, and the extent to which 

 the immersion of the stem in the water is increased is noted. 

 The solid body is then removed from the platform M and 

 attached to the hook K, and the hydrometer is again im- 

 mersed in the water. When equilibrium of flotation has been 

 established, the immersion of the stem is again read on the scale. 



Let the former of these two numbers be expressed by a and the latter by J), a and 

 h are lengths of a cylinder of uniform diairieter and of circular section ; therefore the 

 volumes of these cylinders are proportional to their lengths ; and, as the same liquid is 

 displaced in each case, the weights of the liquids so displaced are also proportional to 



Fig. 1. 



