Ch. X] REAGENTS AND THEIR PREPARATION 347 



potash to 90 cc. of alcohol. But here is a case where the alcohol being 

 of less specific gravity than water the mixture would not weigh 100 

 grams; and to make the mixture weigh 100 grams, giving therefore an 

 exact percentage, one should take 90 grams of alcohol and add to it 

 10 grams of caustic potash. In practice in making solutions of col- 

 lodion or parlodion one usually mixes ether and 95 % or absolute 

 alcohol in equal volumes and then for a 10 % solution 10 grams of the 

 dry soluble cotton or parlodion are added to 90 cc. of the ether-alcohol 

 mixture. But ether is much hghter than water and the alcohol 

 somewhat hghter, so that the percentage in this case would be more 

 than 10%, because the 90 cc. of alcohol and ether would weigh con- 

 siderably less than go grams. 



§ 537. Mixtures of liqtxids to obtain a desired percentage. — It 

 frequently happens that it is desired to obtain a lower percentage or 

 strength of a hquid than the one in stock. This is very readily done 

 according to the general formula: Divide the percentage of the strong 

 solution by the percentage of the desired solution and the quotient 

 will show how many times too strong the stock solution is. 



To get the desired strength, use i volume of the strong stock solu- 

 tion, and add to it enough of the diluting liquid to make a volume 

 corresponding to the amount indicated by the quotient obtained by 

 dividing the percentage of the stock solution by that of the desired 

 solution. For example, if it is desired to obtain a 5% solution of 

 formaldehyde from a stock solution of 40 % strength, the stock solu- 

 tion being 8 times too strong, to get the 5 % solution i volume of 

 the strong solution must be used and 7 volumes of the diluting 

 liquid (water). The solution so obtained will be | of the original 

 strenglth, or 5%. 



If a 2 % solution were desired then i volume of the strong solution 

 would be taken and 19 volumes of water, etc. 



§ 538. Mixtures of alcohol. — For alcohol if one desires a 50 %, 

 solution it is usually near enough correct to add e^ual parts of 95 % 

 alcohol and water, but this does not actually give a 50% solution. 

 To find the real proportions according to the general formula: 95 % -^ 

 so % = 1.9, i.e., for every i cc. of 95 % alcohol should be added 0.9 cc. 

 of water or for each 100 cc. of 95 % alcohol, 90 cc. of water. This 



