PllOCEEDINGS or GEOLOGICAL SOCIETIES. 
107 
To account for the advent and subsequent disappearance of such vast 
masses of ice, various hypotheses have been propounded. It has been 
suggested that the temperature of space is not uniform, and that our solar 
system, in performing its proper motion among the stars, sometimes passes 
through regions of comparatively low temperature : according to this hj^- 
pothesis, the glacial epoch occurred during the passage of our system 
through such a cold portion of space. Some have imagined that the heat 
emitted by the sun is subject to variation, and that the glacial epoch hap- 
pened during what may be termed a cold solar period. Others, again, be- 
lieve that a different distribution of land and water would render the cli- 
mate of certain localities colder than it is at present, and would thus suffi- 
ciently account for the phenomena of the glacial epoch. Finally, Professor 
Xiimtz considers that at the time of the glacial period the mountains were 
much higher than at present — Mont Blanc 20,000 feet for instance — the 
Secondary and Tertiary formations having been since eroded from their 
summits. 
The two last assumptions are attended with formidable geological diffi- 
culties, especially when it is considered that the phenomena of the epoch 
in question extended over the entire surface of the globe ; they have 
therefore never acquired more than a very partial acceptance. With regard 
to the two first-named hypotheses, my colleague. Professor Tyndall, has 
recently shown that they are founded upon an entirely erroneous concep- 
tion of the conditions necessary to the phenomena sought to be explained. 
The formation of glaciers is a true process of distillation, requiring heat as 
much as cold for its due performance. The produce of a still would be 
diminished, not increased, by an absolute reduction of temperature. A 
greater differentiation of temperature is what is required to stimulate the 
operation into greater activity. Professor Tyndall does not suggest any 
cause for such exalted diflerentiation during the glacial epoch ; but he 
proves conclusively that both hypotheses, besides being totally unsupported 
by cosmical facts, are not only incompetent to constitute such a cause, but 
aiso assume a condition of things which would cut off the glaciers at their 
source, by diminishing the evaporation upon which their existence essen- 
tially depends. 
The speaker divided the great natural glacial apparatus into three parts 
— viz. tlie evaporator, the condenser, and the receiver. The part per- 
formed by the ocean as the evaporator is too obvious to need description. 
The two remaining portions of the apparatus, however, are generally con- 
founded with each other. The mountains are in reality the receivers, or 
icehearers, and are only in a subordinate sense condensers. The true con- 
denser is the dry air of the upper region of the atmosphere, whicli permits 
of the free radiation into space of tlie heat from aqueous vapour.* 
All the hypotheses hitherto propounded having therefore failed, m the 
light of recent research, to account for the conditions which brought about 
the glacial epoch, the speaker felt less reluctance in advancing a new the- 
ory, which had gradually elaborated itself out of the impressions he had 
received during a recent visit to Norway. Any such theory must take 
cognizance of the following points in the history of the glacial epoch :— 1st, 
coloured drawings of these remarkable objects, taken from the sketches of Professor 
James D. Forbes and Mr. Matlieu Williams. . . 
* This radiation from aqueous vapour was experimentally shown by causing a jet or 
dry steam to pass in front of, and at a distance of two feet from, a thermo-electric pile ; 
the galvanometer connected with the latter promptly showed a large deflection lor heat, 
proving that the pile was receiving radiant heat from the acjucous vapour. A jct ot air 
heated in the same manner and projected in front of the pile produced no such etiect. 
