353 
Fig. 1.—Spectra of the light transmitted by aqueous solutions. 
Red end. Blue end. 
1, Aphideine. 
2. First change. 
i 
3. Second change. 
Fraunhofer’s lines. 
The addition of a small quantity of citric acid immediately 
alters the colour to yellow, and then the spectrum merely 
shows an absorption of the blue end, extending to about the 
centre of the green, without any definite absorption-bands. 
A little ammonia restores the colour to its original state, and 
therefore the crimson colour is characteristic of a neutral or 
slightly alkaline solution. When a small quantity of the 
double sulphate of protoxide of iron and ammonia is added 
to the solution in its natural state (as in all similar cases, 
using along with it some of the double tartrate of potash and 
soda, to prevent the precipitation of oxide of iron), it is 
changed at once to a pale flesh-colour ; and, if a little am- 
monia had been previously added, the solution becomes quite 
colourless. On exposure to the air, it changes back again to 
the original tint, from the surface downwards. No such 
alteration is produced by adding the ferrous salt to an acid 
solution. ‘This red substance, therefore, like hemoglobin and 
hematin, exists in an oxidised and in a deoxidised condition, 
and, like them, can be deoxidised by the above-named pro- 
cess only when the solution is somewhat alkaline. It thus 
seems reasonable to suppose that it may perform the same 
functions in the economy of those insects which contain it 
that hemoglobin does in the case of the vertebrata. For con- 
venience, it may be well to'call this red colouring matter of 
Aphides Aphideine. It is entirely different from any 
substance on which they feed, and is the same in several 
species living on entirely different plants. 
One of the remarkable peculiarities of hemoglobin is that 
it can be changed into a number of substances, each giving a 
