70 The Microscope. 



theory to pathology. But as first announced by Schwann, the 

 theory has been variously modified owing to further researches 

 in embryology, histology, and especially in cell-multiplication 

 and the formation of multinucleated cells. (In the first paper of 

 this series Hooke is said to have discovered the cell in 1865; it 

 should have read 1665.) 



CARBON DYES. 



PROFESSOR WILLIAM H. SEAMAN. 



THE scientific conception of color is a wave of a certain defi- 

 nite length in the cosmic ether. The name is also given to- 

 any substance that has the property of reflecting or transmitting^ 

 particular waves, and can be applied in any way to a surface to 

 which it gives its own color. 



Substances of the latter class are of two kinds, those which 

 must be stuck on or fastened by some adhesive cement to the 

 surface to which they are applied, and those which soak into- 

 and saturate the material. The first are usually insoluble pow- 

 ders, known as pigments, and the latter are soluble in a medium 

 such as water, alcohol, or benzene, and are called dyes. The 

 latter also combine chemically more or less with the substances 

 to which they are applied. 



Previous to 1856 the latter were mostly of vegetable origin, as 

 logwood, fustic, quercitron, etc. In that year Perkins' violet was 

 put on the market, the first dye commercially successful that was 

 made from anilin, the latter substance being obtained indirectly 

 from coal tar, which has also furnished the raw material for 

 many other colors, often called generically, coal tar colors. 



These substances have proved superior in every respect as 

 dyes to any others known, and hence will in a short time be the 

 only dyes in use. The popular idea, that they are fugitive, is 

 based on some of the first, and is not true. They differ among^ 

 themselves, just as do the older dyes, in this respect, but many 

 are the fastest colors known. Excepting carbon black, every 

 color is modified by long exposure to light, moisture, and 

 warmth\ 



It is impossible to give any correct ideas about carbon dyes 

 without some reference to their chemical relations, because they 



1 De Abney. Sci. Am. Supplement, No. 689-690. Light and color. Report British 

 Ass'n 1888. A. Kicliardson. Permanence of colors. 



