SPECIFIC GRAVITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 29 



§ 11. Preparation of Accessory Weights. — We have now to consider the prepara- 

 tion or manufacture of the small weights to be placed on the top of the stem in 

 order to produce small increments of the immersion of the hydrometer. They are 

 made of wire. This is wound into spiral cones for the heavier and into I'ings for 

 the lighter weights. The lighter weights are made of aluminium and the heavier 

 ones of brass. 



Generally a set of weights consists of aluminium spirals weighing 0"2, 0"5, and 1*0 

 gram, and rings of the same metal weighing 0"2 and 0"1 gram, also rings 0"05 and 0"02 

 gram. The brass weights are rings of 0"5 and 1*0 gram and spirals of 1, 3, 5, and 

 7 grams. At every operation I aim at making a series of nine independent observ^a- 

 tions of the displacement. In the first observation the lightest added weight is 

 used and the reading (Kj) is near the zero of the scale. The succeeding observations 

 are made while the added weight is increased by O'l gram between each observation 

 of the series. The observations thus obtained are spread over the whole of the scale 

 on the stem. 



The weights may be made so that their nominal weight is their true weight in vacuo, 

 but, as they are always used in air, it is preferable to adjust them by balancing them 

 against standard weights in air. The standard weights exert their nominal vertical 

 pressure only in vacuo, at the sea-level in latitude 45° ; but we have assumed that we 

 are in fact working at the sea-level in latitude 45°, therefore the nominal pressure of the 

 standard weight is affected only by the density of the medium in which it is immersed. 

 When we are actually working in a vacuum the density of the medium is insensible ; 

 when we are working in air its density is ascertained by observation. Our standard 

 weights, which have been verified at Kew, are made of brass (gilt) for weights of 

 1 gram and upwards, and of platinum for weights under 1 gram. The weights destined 

 for use on the stem of the hydrometer are also made of brass for those of 1 gram and 

 upwards, and for those of 1 gram and under they are made of aluminium. There 

 are gram weights and half-gram weights of both brass and aluminium. 



We will consider [a) the preparation of a gram weight of brass as balanced against 

 a standard gram of brass ; and (6) the preparation of a gram weight of aluminium as 

 balanced against a standard gram of platinum. 



(a) As we are dealing with only one kind of material, it is sufficient to equilibrate 

 our weight of brass wire against the brass standard gram in air of known density to 

 obtain a weight which in vacuo exerts a vertical pressure equal to that of the standard 

 gram, and it must exert the same vertical pressure as does the standard gram in air of 

 the same density. Taking the specific gravity of brass wire at 8*38, 1 gram of it displaces 

 O'l 19 cubic centimetre of air, which, at 1*2 milligram per cubic centimetre, weighs 0*1428 

 milligram. Therefore, when reckoning the effective pressure exerted by the brass weights 

 placed on the top of the hydrometer in air of the density above specified, we make a 

 deduction from their nominal weight in vacuo in the proportion of •1428 milligram per 

 gram used. 



