SPECIFIC GRAVITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 155 



Section XL — The Principle and Construction of the Open Hydrometer. 



§ 79. AVhen the hydrometer is closed its mass cannot be diminished, and it can be 

 increased only by external additions, which in practice must be immersed either in air 

 or in the liquid. The use of submerged weights is attended by so much inconvenience 

 that it has to be avoided ; consequently, when the instrument is closed, its mass is 

 increased only by adding weights at the top of the stem. The extent to which such 

 additions can be made depends on the stability of the instrument when floating in the 

 experimental liquid. The instrument (No. 0) which I used exclusively during the 

 voyage of the Challenger weighed, in round numbers, 160 grams, and the greatest 

 weight which had to be added to it was 4*071 grams, which produced no disturbing 

 effect whatever. But I had the curiosity to find out what was the limiting weight 

 which could be used without altering the " trim " of the instrument, and it turned out 

 that it could be used in solutions of chloride of sodium of all concentrations. Taking 

 the specific gravity of the saturated solution as 1*2, 160 cubic centimetres of it must 

 weigh 192 grams, so that the weight to be added was 32 grams. The instrument bore 

 it ; but in a solution of chloride of calcium of slightly greater density it took a "list." 

 These experiments showed that the efficiency of the hydrometer did not diminish when 

 the concentration of the solution in which it was used was increased, but its handiness 

 was afiectcd when such heavy weights had to be attached to the stem. 



In order to be able to use a method of such high precision for the determination of 

 the density of solutions of all concentrations, I determined to construct hydrometers 

 which should be left open at the top, so that their internal load, or ballast, could be 

 altered, and they could then be used in exactly the same way as the closed instrument, 

 by adding series of moderate weights to the top of the stem to produce corresponding 

 series of immersions or displacements. 



§ 80. The glass instrument is made after the ordinary pattern, fig. 4, consisting of a 

 spherical bulb at the lower extremity to hold the ballast, a cylindrical body having a 

 diameter not less than that of the ballast bulb, and above it the cylindrical stem of 

 relatively small calibre ; it is left open instead of being hermetically sealed as in the 

 ordinary hydrometer. Instruments of this pattern may be ballasted either with mercury 

 or shot. The latter is the material which has been generally used, because it is more 

 easily handled than mercury, and, being confined in a spherical bulb of relatively small 

 size, it cannot shift and thereby disturb the trim of the floating instrument. Moreover, 

 when shot of a given and uniform size — for instance, No. 10 — is used, the load may be 

 altered and adjusted by counting pellets. 



When we have to deal with concentrated solutions of salts which are at once very 

 soluble and very expensive, the lower ballast bulb is suppressed, and the ballast is 

 accommodated in the cylindrical body of the instrument, as in fig. 5. With this not 

 uncommon form of instrument, and a cylinder of no greater diameter than that which 

 is absolutely necessary to secure free flotation of the hydrometer, the specific gravity 



