A FEW REMARKS ON BLEACHING POWDER. 521 



The effect of adding the chloride of lime solution to that of the iron 

 salt, is to throw down a red powder (sesqui-oxide of iron). If, after the 

 first addition of chloride of lime solution be made to the iron, one drop 

 of the mixed solutions be now withdrawn on the end of a glass rod and 

 brought in contact with the drops of the red prussiate on the white 

 plate, there will immediately be formed a dark blue precipitate (Prussian 

 blue). This indicates that the iron solution has not had enough of the 

 chloride. A further small addition of the solution of bleaching powder 

 is now made. Another drop of the mixture is withdrawn and laid on 

 the prussiate solution as before — if there is still a dark blue precipitate 

 produced, a further addition of the chloride of lime is needed. During 

 the process this solution should be shaken up. The estimation of the 

 chlorine is known to be complete when a drop of the mixed solutions 

 no longer gives a dark blue precipitate with the prussiate drops on the 

 white plate, but gives, instead, a green colour with little or no precipi- 

 tate. When, after cautious additions of the chloride solution, this point 

 is reached, the number of measures which it has taken to effect the 

 change are now read off from the graduated tube. A very simple cal- 

 culation now follows, which is based on the fact that the portion of the 

 50 grains of the sample now used in oxidising the solution of iron (for 

 it is a process of oxidation), contains exactly 10 grains of chlorine 

 available for bleaching purposes. The first step in the calculation is 

 to divide the number of measures now used by 2. The reason of 

 this step is obvious ; because, as the 50 grains of the sample were 

 diffused through 100 measures, each measure corresponds to half a 

 grain of the sample. The quotient of dividing by 2 will, therefore, 

 give the number of grains of the sample, containing 10 grains of 

 chlorine. This number is now made the first term in a simple pro- 

 portion ; 10 the second, and 100 the third ; and the result of this calcu- 

 lation is the percentage of chlorine in the sample. If the steps in 

 this simple calculation be reflected on as they are worked out, it will be 

 seen that the several steps might be combined into this one formula. — 

 Divide 2,000 by the number of measures of chloride solution used. — 

 An example will now make it all clear. If, in any sample, it takes 

 70 measures to oxidise thoroughly the iron solution, the half of 70 is 35, 

 and as 35 is to 10, so is 100 ; the answer is, 28*57 per cent. A result 

 which would be more easily deduced by dividing 2,000 by 70 ; when, 

 as before, 28 - 57 would be shown to be the percentage amount of 

 available chlorine in the sample supposed. 



Though a description of this really excellent and trustworthy pro- 

 cess cannot be well condensed into fewer words, yet it must not be 

 thought tedious in its execution ; because four or five times trial of it, 

 by way of practice, is enough to render one sufficiently expert to over- 

 take the testing of half-a-dozen samples in an hour or two ; and, we 

 may add, that the information gained by the process must be coupled with 

 the satisfaction of having performed it all oneself. — Pager Trade Review. 

 vol. in. y y 



