963 
of Edinburgh, Session 1885 - 86 . 
molecular weight, and therefore of most rapid vibration — like the 
ultra-violet rays of the spectrum — are incapable of producing sensa- 
tion. In like manner there are substances of very great molecular 
weight, and of the most sluggish molecular vibration, like the vibra- 
tions of the ultra-red, also incapable of effecting the sensorium. 
Let us turn again to our study of sapid substances, restricting 
ourselves in the first case to the investigation of inorganic com- 
pounds. A large number, perhaps the greater number of inorganic 
compounds, are insoluble in water, and are therefore tasteless, so we 
are confined to those which are soluble, which are chiefly the salts 
of certain metals. 
Many elements are so closely related that they may he classed to- 
gether. New and curious relationships have recently been discover- 
ed by Newlands, Lothar Meyer, Mendelejeff, and Carnelley. In the 
groups of elements shortly to be mentioned we find the presence of 
common physical and chemical properties. It has been shown by 
Dr Lauder Brunton that elements with similar physical properties 
have similar actions on protoplasm generally. It will be my endea- 
vour to show that they are capable in the same way of stimulating the 
end-organs of the gustatory nerve, and producing similar sensations. 
The nature of the Periodic Law is now so well known, thanks to 
the many recent publications of Professor Carnelley, that it would 
be superfluous to attempt more than roughly to sketch its main 
features. If we arrange the elements in the order of their atomic 
weights, beginning with that which has the lowest, and passing to 
that which has the highest, we shall find that there is a periodical 
recurrence of function or property in the series. The first element 
is a monad, the second a dyad, the third a triad, and the fourth a 
tetrad. Then we find the fifth a triad, the sixth a dyad, and the 
seventh once more a monad. Then follows a second series of seven 
elements, showing the same variation in atomicity, and this repeats 
itself right through the list of elements. This periodic recurrence 
is seen not only with the atomicity, but with the atomic volume, 
the fusibility, the electrical and other properties of the elements. 
There is then a general resemblance in physical properties between 
the first, eighth, fifteenth, &c. ; between the second, ninth, sixteenth, 
&c., and so on. Mendelejeff has arranged the elements in a very 
convenient tabular form, which brings out these and some other 
important facts. 
