QUANTITATIVE ANALYSIS.] 



CHEMISTRY. 



405 



precipitate. In employing the sand-bath, care should be 

 taken, if a filter is being dried, that it do not attain a 

 temperature sufficient to char it, as that would occasion 

 a loss of weight, owing to the decomposition of the 

 paper. 



When a filter is very dry it also becomes rigid ; and 

 the.least touch may break it, or cause the dry powder 

 it contains to jerk out. This is a point requiring 

 attention and care. We have seen young students 

 destroy all the hoped-for reward of their previous at- 

 tempts, for want of delicacy of touch and manipulation 

 in this respect. 



In some instances, the only result obtained which is, 

 generally speaking, far from being satisfactory is that 

 of crystalline product. Under this head the alkalies 

 must be placed ; as, with little exception, no solid pre- 

 cipitate can be obtained from which they can be esti- 

 mated. Potass, for instance, may be weighed as a sul- 

 phate, nitrate, &o. Soda may be similarly estimated. 

 In such, and many other similar instances, when crys- 

 tallisation is had recourse to, it must carefully be noticed 

 whether the crystals are likely to possess water as a con- 

 stituent. We do not here refer to water in a liquid 

 ftate, but in chemical combination. In some cases 

 crystals lose their water of crystallisation, and from this 

 cause serious loss may occur in the estimation of the 

 base. The water may not be driven off at the boiling 

 ;rature ; hence a water-bath may be used. Gene- 

 rally speaking, all such salts, however, suffer loss when 

 the temperature is raised beyond that point ; and 

 the sand-bath must therefore be employed with great 

 caution. 



Presuming that the student has mastered the details 

 through which we have been proceeding, his last step 

 will be that of finding the relative value in respect to the 

 chemical equivalents of bases, <tc. , of the products which 

 he has obtained. This, however, is not a question of 

 very great difficulty ; because the simplest rules of 

 arithmetic alone are involved. In fact, the Rule of 

 Three or Proportion is that which is to be employed, 

 and of which we shall give some examples. 



Suppose, for example, that in Experiment 82, p. 397, 

 in our remarks on qualitative analysis, 100 grains of 

 marble had been employed, and that after dissolving the 

 solid in hydrochloric acid, of which a known weight had 

 been added, it was found that 44 grains had been lost 

 Now, as by the subsequent part of the process it was 

 discovered that the gas which had escaped was carbonic 

 acid, and that the solid with which the acid was in com- 

 bination was lime, we wish to know how much lime was 

 actually present in 100 grains of marble. As that sub- 

 stance lost 44 grains of carbonic acid, and as 28 grains of 

 lime unite with 22 grains of carbonic acid to form 50 

 grains of marble, we should have the following pro- 

 portions : 



One equ'Ttlcnt The amount of CO a 

 AS at COg. in 1UO (mini of 



22 



marble. 



44 



One equivalent of 

 lime, CaO 



! 28 



Now multiplying 44 by 28, and dividing by the first 

 term as in ordinary Proportion, we obtain 



44 

 28 



352 

 88 



22) 1232 



M 



Hence there will be 56 grains of lime in 100 grains of 

 marble. Of course this long sum was not required to 

 prove the fact ; as it is evident that there will be twice 

 as much lime in 100 as in 60 grains of marble ; but we 

 give this easy illustration that our readers may under- 

 stand the process. 



We will give another illustration, which, although on 

 the same principle, yet involves a little more apparent 

 difficulty. \Vo will suppose that an alloy has been dis- 



solved in nitric acid, and that the silver has been pre- 

 cipitated as chloride of silver, of which there are 48 

 grains obtained ; how much chlorine and silver are indi- 

 vidually in that quantity ? 



Now the equivalent of chloride of silver is 



Chlorine 36 



Silver 103 



Or 144 



And we must accordingly make two proportions to as- 

 certain the amount of each element present. 

 Commencing with the silver, we say 



ArCl ArCl 4r Ar 



144 : 48 : : 108 : 36 as below. 

 48 



864 

 432 



144) 5184 (36 grains of silver. 

 432 



864 

 864 



In this the 48 is placed in the second term of tho pro- 

 portion, because the answer must necessarily be smaller 

 than the third term. 



With the chlorine we say 



Ar Cl Ar Cl Cl Cl 



144 : 48 



36 

 48 



288 

 144 



12 grains as 

 below. 



144) 1728 (12 grains of chlorine. 

 144 



288 

 988 



Now, by the first proportion, we find of silver 36 grs. 

 ,, by the second proportion, of chlorine 12 ,, 



48 



Hence we find the total equal to that of the number 

 of grains found in the precipitate. 



Wo shall now take another case ; in which, having 

 ascertained, as above, the exact amount of each sub- 

 stance in a precipitate, the per-centage of each, or its 

 proportion in one hundred grains, is desired. This is a 

 very common question, especially in assaying and me- 

 tallurgical processes generally. 



The method to be adopted is precisely similar to that 

 illustrated above ; and we will take our lust example to 

 show the modus operandi. 



Supposing that the 48 grains of chloride of silver were 

 precipitated from 600 grains of a lead ore containing 

 ln'v much per cent, of silver does the ore contain / 

 We state it as 



two : 36 : : 100 : 6 per cent, as below. 

 30 



COO) 3600,(6 



3000 



Hence such an ore would contain 6 per cent, of metallic 

 silver; or, in other words, every 100 grains of the ore 

 will afford six grains of pure silver. 



Occasionally, the proposition is in the reverse form ; 

 that is, the per-centage being given required the amount 

 in any number of grains ; say, for instance, in 000. Then 

 we state it 

 j 100 : 6 : 600 

 6 



100 



3000 



36, or 36 grains in COO of the ore. 

 Our readers not well conversant with arithmetic, must 



