JOHN TYNDALL 245 
spontaneous generation, and materially helped the 
new and epoch-making germ theory of disease. 
Another grand division of Tyndall’s work relates to 
radiant heat. His work on this subject began in 1859, 
and was kept up during the greater part of his life. 
Perhaps the most important part of it was comprised 
in his researches on the transmutation of the dark heat 
rays below the red end of the spectrum and their rela- 
tions to the luminous rays. But upon these and sun- 
dry points in optics and acoustics to which Tyndall 
made notable contributions I do not feel competent to 
speak. 
Among those of Tyndall’s books which have a place 
in literature as well as in science, “ Heat considered as 
a Mode of Motion” is doubtless the most eminent. At 
the time when it was published, in 1863, the doctrines 
of the correlation of forces and the conservation of 
energy were still among the novelties, and the re- 
searches of Joule, Helmholtz, and Mayer, which had 
done so much to establish them, were not generally 
understood. Tyndall’s book came in the nick of 
time; it was a masterpiece of scientific exposition such 
as had not been seen for many a day; and it did more 
than any other book to make men familiar with those 
all-pervading physical truths that lie at the bottom of 
the doctrine of evolution. This book, moreover, 
showed Tyndall not only as a master in physical 
investigation, but as an eminent literary artist and one 
of the best writers of English prose that our age has 
seen. 
Tyndall’s other direct connections with the exposi- 
tion of evolution have consisted mainly in detached 
statements of special points from time to time in brief 
