Page:Proceedings of the Royal Society of London Vol 60.djvu/242

Rh in air. Prom the modification of their appearance, and the measurements originally made from them, their identity was not quite apparent, although probable.

It thus appears that, with the spark, the cyanogen spectrum is nothing like so strongly marked, as is the case with the flame of the gas, only one group of bands being represented, and that when the spectrum is taken in air the cyanogen does not appear, because in all probability the substance is oxidised.

I have already stated that the formation of cyanogen which yields the characteristic spectrum is a synthetical operation, that compound substances, when burnt in flames, do not, as a rule, emit the spectrum of the compound, but the spectrum of one or more of the elements of which it is composed, or that of one or other of its products of combustion.

How then are we to account for the cyanogen spectrum in the flame of burning cyanogen ?

The conditions under which combustion takes place are these: there is an excess of the gas, the temperature of the flame is exceedingly high, and the gas within it is not in contact with a solid substance, hence immediate decomposition does not occur, and the gaseous compound is heated to incandescence.

The following measurements were undertaken at the proposal of Professor W. F. R. Weldon, and to him I am greatly indebted for many suggestions, and for the kindly help he has always so readily given me.

The crabs were obtained from the Biological Station at Plymouth, and sent at intervals during a period of about two years, dating from the autumn of 1893. Only males were measured. Seven measurements were made on each individual, corresponding to those made by Professor Weldon on the female of Garcinus moenas (‘ Roy. Soc. Proc.,’ vol. 54).

1. Carapace length, AB (fig. 1). 2. Total carapace breadth, CC'. 3. Frontal breadth, DD'. 4. Right antero-lateral, AC. 5. Left antero-lateral, AC'. 6. Right dentary margin, CD. 7. Left dentary margin, C'D'.