APPENDIX A LI 
worthy of the present age, as compared with former times, may seem 
to justify such an attitude. Very slow was the progress cf the science 
of electricity and its applications up to the middle of last century. 
Scheele observed the blackening of silver chloride by the action of light 
in 1779. It was sixty years after this discovery that it had its applica- 
tion in the camera, and forty years more before the dry plate made 
photography generally serviceable. Compare this with the many 
striking achievements of science during the last quarter of a century 
and the rapidity with which they are applied to use. 
The acceleration is due in part to the extensive stock of scientific 
knowledge which has been already accumulated, but more especially 
to the large number of persons who are engaged in science or scientific 
pursuits. It may be assumed, under the law of averages that out of 
the total number of these persons at any time, a certain percentage 
possess minds qualified to initiate ideas which will advance science. 
As the total number increases, the number of these and the number 
of valuable ideas which they will develop, and therewith the stock of 
knowledge, will increase. 
Possibly the natural increase of knowledge, after this fashion, 
may proceed faster than merely in proportion to the number of workers, 
for the increasing stock of knowledge affords to the leaders of thought 
a wider foundation on which to build their generalizations; but in view 
of the many problems that are pressing for solution, still more rapid 
progress would be desirable. It is not advisable to leave the advance 
of science to the chances of averages, or to sporadic efforts. The aim 
should be to develop the latent powers of the many potential discoverers. 
Of problems of vital interest to all, two may be mentioned. The 
pressure of population on the means of subsistence has hitherto been 
retarded by advances in the science and practice of agriculture as well 
as by the opening up of vast areas of fertile land; but the latter check 
will not continue to operate for an indefinite time. In fact even now 
it seems to be insufficient, as the increasing cost of food testifies. War, 
pestilence and famine have been called natural checks to population. 
Of these the first two have been restrained, and are we hope on the way 
to being abolished. Thereby population increases faster and greater 
necessity arises for increase in the food supply, for which we must look 
to advances in agriculture. 
Again for the power that drives our machinery we depend chiefly 
upon the energy which is furnished by the heat of the sun, and stored 
up in fuel, or supplied by falling water. The forests are being depleted 
both by use and by waste. In burning coal and oil the world is using 
up its capital, inherited from remote geologic ages. The disappearance 
of the forests is destroying the water powers. 
