18 
mineral lode, the alteration in the character of the lode or 
vein after a dislocation has taken place, the generally uniform 
direction of lodes, and many other phenomena which it will fall 
to the province of my colleague, Mr. Warington Smyth, to 
describe, appear to show the probability of such a power as 
electricity, in some of its modifications, being the exciting agent. 
This is one of those extensive subjects which yet remains open 
for the investigation of an intelligent and industrious observer. 
In a course of lectures which is to embrace a consideration 
of the physical forces the phenomena of the solar radiations 
cannot be omitted. The practice of photography belongs to 
these, and furnishes another choice example of my text, — the 
value of observation. Omitting, however, all mention of the 
useful applications of photography as a means of making 
philosophical instruments self-registering, allow me to state that 
a careful investigation of the thermic influences of the solar 
spectrum, associated with a physical analysis of various trans- 
parent media, led me to the discovery of a glass, but slightly 
coloured green, which possesses the property of separating and 
keeping back a class of solar rays remarkable for their scorching 
power, and which, without obstructing any of the necessary 
radiations, thus protects the tropical plants in the great con- 
servatory of the Royal Botanic Gardens of Kew from the 
injurious influences of scorching sunshine. 
The solar spectrum exhibits several distinct phenomena. 
Tight, heat, and chemical action are evident; and these give 
rise to electrical disturbance. Beyond these a class of radia- 
tions have been detected, which have the combined action of 
the heating and chemical rays, but which are in many respects 
dissimilar to either. These rays increase in quantity relatively 
to the other principle as the seasons advance ; they being most 
abundant in the autumn ; when, in all probability, they perform 
an important part in the ripening of fruit. These are the rays 
which, it has been found, operated to produce a kind of scorch- 
ing on the leaves of those plants which grow in houses glazed 
with a white glass. To obviate this, was the problem submitted 
to my care ; and the suns of two summers and autumns have 
shown that my experiments did not deceive me. No case of 
