972 
Proceedings of the Eoyal Society 
We see here again that, with a definite molecular structure, with 
the presence of certain groups of elements, a definite taste sensation 
is produced. A chemist examining the rational formula of a sub- 
stance will predict its properties to an extent which will vary with 
the extent of his knowledge. We ton can predict a property, that 
of producing a particular sensation, when applied to the tongue. 
There is a large number of organic compounds having a hitter 
taste. Such is quinine, quassia, strychnia, &c., but inasmuch as 
so little is known as to the chemical nature of these substances their 
investigation would he of little use. 
We have seen that in the inorganic compounds the physical, 
chemical, and taste properties vary with the nature of the elements 
in the sapid substance ; with similar elements similar properties are 
associated. So, too, we have seen among the carbon compounds 
that the physical, chemical, and taste properties vary with the com- 
pound radicals present in the sapid substance. The question now 
to he determined is, whether or no the compound radicals behave 
as elements, vibrating in a characteristic and definite way, not 
materially disturbed by altering the combination. This may he 
determined by the eye, with or without the aid of the spectroscope. 
We know that chromic acid has an orange-red colour; combine it 
with metallic elements to form chromates, and we find that these are 
coloured, the colour varying very slightly with the metal. 
Picric acid is a coloured solid, and it probably owes its colour, at 
any rate in part, to the presence of a radical (N0 2 ). Its salts, too, 
are coloured, the absorption shifting towards the red end of the 
spectrum. In his interesting researches into the spectra of colour- 
less fluids, observed through a considerable length of tube, Dr W. J. 
liussell finds that ammonia gives certain well marked absorption 
bands. If an atom of hydrogen in the ammonia be replaced by 
methyl forming a substance methylamine, the ammonia bands are 
still seen, shifted, however, slightly towards the red end of the 
spectrum. 
H 
i 
H 
i 
H 
i 
1 
N-H 
i 
1 
N- 
i 
1 
C-H 
i 
1 
H 
1 
H 
1 
IT 
Ammonia. Methylamine. 
