Measuring the Water of Crystallization of Salts. 211 



full contains exactly 500 grains of pure water at the tempe- 

 rature of 60^ F. I balance it on a pair of scales, then take a 

 solution of the salt made by heating water with more than it 

 can dissolve, allow it to cool to the required temperature, so 

 that it may be perfectly saturated, and fill the bottle exactly 

 half-full of the solution. The bottle being half-full is then 

 balanced on a pair of scales, and filled up to the stopper 

 with the salt well-dried and in fine powder: it is then care- 

 fully weighed again, and this weight, minus the weight of the 

 former additions, is divided by 250, which gives the specific 

 gravity of the salt. The reason I divide by 250, is because 

 the bottle will hold 500 grains volume of pure water; there- 

 fore if it be half-full of solution and hall-full of salt, 250 will 

 equal the volume of that salt compared to an equal volume of 

 water; so that if a saturated solution, alcohol, aether, or any 

 other fluid, be poured into the bottle till half-full, there will 

 be a bulk which will equal 250 grains of pure water. 



On referring to Dr. Ure's Dictionary of Chemistry, I found 

 several of the specific gravities of the salts used in my expe- 

 riments, and these agreed almost exactly with the results as 

 determined by the above method, though I afterwards found on 

 repeating the experiments, some of the others, especially the 

 hydrous salts, to be incorrect. Thus, according to my expe- 

 riments, I found the specific gravity of nitrate of potash to equal 

 2"075, while the specific gravity of the same salt, according to 

 Dr. Ure, was 2-073, and several of the others agreed almost 

 as closely ; I therefore inferred that the preceding method 

 was correct. 



It is important to use a satui'ated solution of the substance, 

 the specific gravity of which is to be taken ; for as I afterwards 

 found, there was generally a certain condensation or expan- 

 sion when a salt dissolved. For if pure water be employed, 

 or any fluid that will partially dissolve the substance when put 

 into the bottle, there will be an erroneous result of the expe- 

 riment, amounting in cases where there is a condensation to 

 a greater specific gravity than the truth, and where there is 

 an expansion to a less. Thus on dissolving 100 grains nitrate 

 of potash in 500 grains water, I found the specific gravity of 

 the solution to = I'llOO, while the calculated specific gravity 

 equalled 1*094<4, for 100 grains nitrate, specific gravity 2*074', 

 equal 48'21 grains of pure water bulk for bulk, and 500 grains 

 of water equal 500. Therefore 



100 + 500 = 600 for the weight, 

 which divided by 48*2 1 + 500=54'8-21 for the bulk of water, 

 gives roo^^ as the specific gravity of the solution. But since 

 when 100 grains of nitrate of potash are dissolved in 500 grains 



P2 



