16 



HYDROSTATICS. 



somewhat less than twenty-four ounces, 

 and plunged in water, there is a fall or 

 rise of above half an inch for every 

 1-1 7424th part of the water displaced ; 

 so that a diiference of the hundred - 

 thousandth part is easily perceived. 



The principal use of the Hydrometer 

 is to ascertain the specific gravity of 

 spirits, and to detect adulteration or 

 mixture of water with it. But it is 

 equally applicable to find the specific 

 gravity of any other fluids. Thus milk 

 is about 1-31 part heavier than water; 

 and therefore the instrument will easily 

 shew if water has been mixed with 

 it. Accordingly, in Switzerland and 

 the North of Italy, where the peasants 

 all bring then* milk every evening to a 

 common dairy, and having it measured 

 each time, are allowed a portion of 

 cheese at the end of the season, ac- 

 cording to the quantity of the milk 

 carried in the books to the credit of 

 their account, a Hydrometer is used to 

 detect any mixture of water, which 

 would make the milk lighter. 



By means of the same kind we can 

 ascertain the adulteration of solid sub- 

 stances, as metals, mixed with others 

 of different specific gravity. An in- 

 strument upon this principle was 

 constructed by Mr. Bradford, calcu- 

 lated to shew how much alloy there 

 was in the gold coin, and also whe- 

 ther that alloy was silver or a lighter 

 metal. 



The proposition which forms the 

 foundation of this branch of Hydro- 

 statics, that a solid plunged in a fluid 

 displaces a quantity of the fluid equal 

 to its bulk, was discovered by Archime- 

 des, one of the greatest mathematicians 

 of ancient times, in consequence of 

 Hiero, king of Syracuse, his friend and 

 patron, and himself an eminent philo- 

 sopher, and, it needs hardly be added, 

 a virtuous and patriotic prince, having 

 set him a problem to solve upon the 

 adulteration of metals. Hiero had 

 given a certain quantity of gold to an 

 artist to make into a crown, and sus- 

 pecting, from the lightness of the crown, 

 that some silver had been used in mak- 

 ing it, he begged Archimedes to inves- 

 tigate the matter. It is said that while 

 this great man was intent upon the 

 question, he chanced to observe, in 

 bathing, the water which ran over the 

 sides of the bath, and finding, by cal- 

 culation, that it was equal to the bulk 

 of his body, and immediately perceiving 

 that this furnished him with the means 



of detecting the adulteration, by trying 

 how much water a certain weight of 

 silver displaced, how much a certain 

 weight of gold, and how much a certain 

 mixture of the two, he rushed out 

 of the chamber, exclaiming, " / have 

 found it ! I have found it ! " 



But no test of this kind can ever be 

 accurate either with respect to solids or 

 liquids, unless we have previously ascer- 

 tained by experiment that the substances 

 mixed do not enter into such a che- 

 mical union as alters their internal 

 structure ; for in many cases this takes 

 place, and the specific gravity of the 

 parts in composition is thus different 

 from their specific gravity when sepa- 

 rate. A piece of gold, in which there 

 should be a hollow space filled with 

 silver, could, by the process of weighing 

 in water, be accurately proved ; for w r e 

 should thus ascertain that a given bulk 

 fell short of the requisite weight by a 

 certain quantity. But if the two me- 

 tals were melted together, and che- 

 mically united, they might very pos- 

 sibly form such a compound as should 

 have its specific gravity greater or less 

 than the medium of the specific gravi- 

 ties of the gold and silver ; and so of 

 fluids. Before, therefore, the Hydrosta- 

 tic Balance or the Hydrometer can be 

 relied on as proofs of admixture, trials 

 must be made with the simple sub- 

 stances, and their compounds in known 

 proportions ; and the effects of the mix- 

 ture being thus ascertained in these cases, 

 the weight becomes an accurate test of 

 the degree of adulteration ; because 

 we have learnt what allowance to make 

 for the effects of chemical combination. 



But there is one circumstance which 

 must, in all trials for ascertaining spe- 

 cific gravity, be taken into the account ; 

 and that is, the Heat or temperature of 

 the substances at the time of the expe- 

 riment. The effect of heat is to in- 

 crease the bulk of all bodies ; conse- 

 quently to make them specifically 

 lighter, by making the same quantity 

 of matter fill a larger space ; and differ- 

 ent substances are expanded in different 

 degrees by it. Thus water, when heated 

 from 60 degrees of Fahrenheit's ther- 

 mometer to 100, is increased in bulk 

 1-1 6 7th part, mercury not above 1 -243 ; 

 consequently, a cubic inch of water 

 weighs about a grain and a half lighter 

 (the 167th part of 253, the weight of 

 the cubic inch, when of the ordinary 

 heat of 60 degrees). A cubic inch of 

 mercury, equally heated, weighs 14grs. 



