Mat 18, 1906.] 



SCIENCE. 



791 



know the index of refraction of solid gelatine. 

 As I failed to find the value in any book of 

 constants to whicli I had access I proceeded 

 to make the determination. I took a 90° 

 totally reflecting prism of flint glass and on 

 the long face dropped a few drops of the purest 

 gelatine I could buy — dissolved in warm water. 

 The liquid gelatine spread out and formed 

 two disks of the gelatine about the size and 

 thickness of silver dimes. I then laid the 

 prism away for the gelatine to jlry and harden, 

 expecting to determine its index by determin- 

 ing the critical angle of glass-gelatine. But 

 when I examined the prism a few days later 

 I found it ruined. The gelatine had dried 

 out and contracted, and had clung to the glass 

 with such tenacity that some of the glass had 

 been torn from the remainder of the glass — 

 all the way around the circumference of the 

 gelatine disks, forming an annular cavity 

 from .01 cm. to .1 em. deep. 



This experiment proves, in this instance at 

 least, that the cohesive and contractile forces 

 of gelatine, and the adhesion of gelatine for 

 flint glass are greater than the cohesion of the 

 glass. It proves more, for consider the rela- 

 tively enormous force that must be exerted to 

 pull a piece, say a disk of glass, from (out of) 

 a large plane-faced block of glass where one 

 must take into account the forces about the 

 edge of the disk as well as those on its faces. 



I have lately repeated the experiment, using 

 pieces of window glass instead of the prism. 

 Several times the gelatine disks on drying 

 sprung loose from the glass without injuring 

 the surface. However, on taking extra pre- 

 cautions to have the glass surfaces clean, the 

 gelatine prepared in a clean vessel, and very 

 little water used in dissolving it, patches of 

 glass were pulled oif by the gelatine drying 

 and springing up around the edge of the disk. 



An unsuccessful attempt was made to meas- 

 ure the tenacity of solid gelatine, unsuccessful 

 because of the difiiculty in getting a sample 

 free from internal strain. Further experi- 

 ments are in progress. 



Arthur L. Foley. 



Physics Labobatobt, 

 Indiana Univebsitt, 

 April, 1906. 



NOTES ON ORGANIC CEEMISTRY.' 

 NEW ANALOGUES OF INDIGO. 



The importance of indigo in the arts is so 

 great that considerable general interest at- 

 tends the discovery of related compounds. 

 Some years ago P. Friedlander and J. Neud- 

 oerfer^ prepared a compound which they be- 

 lieved to be represented by the formula, 



Friedlander has now confirmed this result 

 and has also obtained the corresponding thio- 

 derivative. 



H * 



H 



H 



On comparing these formulae with that of 

 indigo, 



it will be observed that the two new com- 

 pounds are to be regarded as indigo in which 

 the bivalent imino (NH) group is replaced 

 by an equivalent atom of oxygen and sulphur, 

 respectively. 



Oxygen-indigo, as the first compound may 

 be termed, is a red dye which is much more 

 rapidly acted on by light than indigo. Its 

 preparation is attended with considerable dif- 

 ficulty. 



Thioindigo, on the other hand, can be ob- 

 tained with comparative ease from thiosali- 

 cylic acid. It also is a red, sparingly soluble 

 dye, crystallizing in brown-red needles with a 

 bronze luster. Its chloroform solution is red, 

 with a shade of blue, and it exhibits a strong 

 yellowish-red fluorescence and a characteristic 

 absorption spectrum. At high temperatures 

 thioindigo is more stable than indigo and may 

 be sublimed and distilled. Thioindigo re- 

 sembles indigo in its behavior with acids, re- 



»Ber. d. Chem. Oes., 32, 1867 (1899). 

 Uhid., 39, 1060 (1906). 



