519 

 A FEW REMARKS ON BLEACHING POWDER. 



BY MURRAY THOMSON, M.D., F.R.S.E. 



This substance is so largely used in paper making, that it may 

 form a subject of considerable interest to many. We have in the follow- 

 ing paper treated the subject in such a way, that it will be simple and 

 instructive to those who may be ignorant of chemistry. 



This widely used substance, as many of our readers may know, was 

 first prepared by Mr. John Tennant, the founder of the St. Rollox 

 Works, Glasgow. And though it has now been in use for upwards of 

 sixty years, it is still as highly esteemed as ever as a bleaching agent. 

 At one time a difference of opinion existed as to what its active con- 

 stituent was ; but now most chemists believe its bleaching power to be 

 due to hypochlorous acid, a compound of chlorine and oxygen. It may 

 be regarded as a mixture of hypochlorite of lime, hydrate of lime, and 

 chloride of calcium, but it is only the first-named substance which is 

 available for bleaching. Another circumstance, not to be lost sight of 

 in this view of the composition of bleaching powder, is that hypo- 

 chlorous acid is destructive of colour, not only on account of the 

 chlorine it contains, but also on account of its oxygen. Indeed it is 

 estimated that this acid has a bleaching power twice as great as the 

 elementary gas chlorine itself. To understand this, our readers 

 should remember that before chlorine and its compounds were employ- 

 ed to effect bleaching, the oxygen of the air, in one or other of its 

 forms, aided by the sunlight and moisture, were the agents used to 

 destroy colour. Keeping this in mind, it is now not difficult to see how 

 a compound like hypochlorous acid should be more energetic as a 

 bleacher, than either of its constituents separately. The method of the 

 action of hypochlorous acid may be perhaps best understood, by saying 

 that its elements, the chlorine and the oxygen, are apt to separate from 

 each other. The tie that keeps them together as a compound is a feeble 

 one, and once unloosed, they are not only free to combine with 

 other substances, but in a state of greater inclination to do so. 

 They are in what the technical language of the chemist calls the 

 nascent state. And when in this condition, they are presented to com- 

 pounds that they can combine with, such as colouring matter, they 

 readily unite, and a new, but now colourless substance, is the result. 



But although it be true that bleaching powder owes its efficacy to 

 both chlorine and oxygen, it is not necessary in estimating its value, 

 to take into account anything more than the amount of chlorine pre- 

 sent ; because the more of it the sample contains, the more oxygen will 

 it also contain, and its bleaching power will be the greater ; and if these 

 follow by equal steps, it is sufficient to know the amount of either 

 element ; and as it is the easiest to ascertain the amount of chlorine, it 

 is always chosen as the element to operate on. 



