ON SELF-RECORDING MAQNETOGRAPHS. 211 



an extensive assortment of waxes of all kinds, and in every degree of purity ; 

 also to Mr. Maskelyne, for a valuable series of the chemical bodies of 

 which the various waxes are composed ; by means of these I have been 

 enabled to examine the effect produced by saturating the paper with 

 bees-wax from different countries, Myrica wax, Canauba wax, China wax, 

 spermaceti, ethal, stearic acid, stearin, palmitic acid, palmitin, paraffin, 

 and various oils. 



22. I find that the action of the wax is purely mechanical, almost the 

 only difference of effect produced by any of the above bodies, widely as they 

 vary in their chemical nature, arising from a difference in their physical 

 properties. 



Stearin, palmitin, and most of the oils, are too greasy in their nature 

 to be advantageously employed. The fatty acids do not make the paper 

 in the least greasy, but they injure the transparency. China wax has 

 almost too high a melting-point, and gives a crystalline structure to the 

 paper. Spermaceti also is too crystalline. Paraffin, ethal, and the waxes, 

 produce very good results ; of these bees-wax is the only one that would 

 be practically available for this purpose. It should be free from stearin, 

 stearic acid, tallow, &c; the presence of a little spermaceti does not much 

 interfere, but as its price does not differ very much from that of pure wax, 

 it is not so common an adulteration as the other cheaper substances. 



23. It will be unsafe to use the wax in the form of round thin tablets, 

 about 4 inches in diameter, in which it is usually met with, as in this state 

 it is generally adulterated to the extent of at least 50 per cent. 



As an article of commerce, it is next to impossible to obtain small 

 quantities of wax sufficiently pure to be relied upon. The only way I can 

 recommend is to apply to one of the well-known large bleachers, and trust 

 to them for supplying the article in a state of purity. Whenever I have 

 found it necessary to make such applications, my request has always been 

 acceded to in the most cordial manner, and every information has been 

 given with the utmost readiness. 



24. The other chemicals (with the exception of the strong nitric acid, 

 which any retail druggist will supply, and the water, which had best be 

 distilled on the premises) should be ordered direct from some manufacturing 

 chemist, as otherwise, unless the operator have a sufficient knowledge of 

 chemistry to be able to detect any inferiority, there is danger of not having 

 the articles sufficiently pure. 



The iodide and bromide of potassium should be ordered purified. 



The nitrate of silver should be crystallized, not in sticks ; it ought to be 

 perfectly dry, and have no smell, acid or otherwise. 



There are usually two varieties of glacial acetic acid to be met with ; the 

 purest must be used ; it should be perfectly free from any empyreumatic 

 odour, and must cause no turbidity when mixed with a solution of nitrate of 

 silver, e. g. in making the exciting bath (42). 



The gallic acid should be as nearly white in colour as possible. 



Especial care should be taken to have the alcohol good ; it should be 60° 

 over proof, and of specific gravity 0'83. On evaporating a few drops on the 

 palm of the hand, no smell should be left behind, nor should it, under the 

 same circumstances, leave any stain on a sheet of white paper. 



25. The hyposulphite of soda will be found one of the articles most 

 difficult to obtain pure ; there is a large quantity at present in the market, 

 having little else of this salt but the name, and being of course totally unfit for 

 use; if there be the least doubt about its purity, it should be tested in the 

 following manner: — ■ 



p2 



