1878.] 
The British Association. 
501 
descending into the arena of special science, to show how 
the most various investigations alike tend to issue in mea- 
surement, and to that extent to assume a mathematical 
phase, I should be embarrassed by the abundance of in- 
stances which might be adduced. I will therefore confine 
myself to a passing notice of a very few, selecting those 
which exemplify not only the general tendency, but also the 
special character of the measurements now particularly re- 
quired — viz., that of minuteness, and the indirect method 
by which alone we can at present hope to approach them. 
An object having a diameter of an 8o,oocth of an inch is 
perhaps the smallest of which the microscope could give 
any well-defined representation ; and it is improbable that 
one of 120,000th of an inch could be singly discerned with 
the highest powers at our command. But the solar beams 
and the electric light reveal to us the presence of bodies far 
smaller than these. And, in the absence of any means of 
observing them singly, Professor Tyndall has suggested a 
scale of these minute objects in terms of the lengths of 
luminiferous waves. To this he was led, not by any attempt 
at individual measurement, but by taking account of them 
in the aggregate, and observing the tints which they scatter 
laterally when clustered in the form of a( 5 tinic clouds. The 
small bodies with which experimental science has recently 
come into contact are not confined to gaseous molecules, 
but comprise also complete organisms ; and the same philo- 
sopher has made a profound study of the momentous 
influence exerted by these minute organisms in the economy 
of life. And if, in view of their specific effects, whether 
deleterious or other, on human life, any qualitative classifi- 
cation, or quantitative estimate, be ever possible, it seems 
that it must be effected by some such method as that indi- 
cated above. Again, to enumerate a few more instances of 
the measurement of minute quantities, there are the average 
distances of molecules from one another in various gases 
and at various pressures ; the length of their free path, or 
range open for their motion without coming into collision ; 
there are movements causing the pressures and differences 
of pressure under which Mr. Crookes’s radiometers execute 
their wonderful revolutions. There are the excursions of 
the air while transmitting notes of high pitch, which 
through the researches of Lord Rayleigh appear to be of a 
diminutiveness altogether unexpected. There are the mo- 
lecular actions brought into play in the remarkable experi- 
ments of Dr. Kerr, who has succeeded, where even Faraday 
failed, in effecting a visible rotation of the plane of polar- 
