Stoney — AjJ^^reciation of Ultra- Visible Quantities. 541 



substances emerge ; but he takes little note of the eventful time 

 during which all the protracted contests of the reaction have taken 

 place, which if it has lasted for only the five-hundred -thousandth 

 of one second has been as long in reference to the activities of the 

 molecules as a long life of 60 years would be in reference to all 

 the thoughts and actions of a man. 



3. The minimum visibile, as defined above, is between the fourth 

 and fifth of a micron, and a speck whose volume is the cube of this 

 may be regarded as the smallest organic speck that the biologist 

 can distinguish from other specks by the highest powers of his 

 microscope. Its volume is accordingly about one-hundredth of a 

 cubic micron — about the 1/7000*^ part of the volume of one blood 

 corpuscle. Now, liquid or solid material, if resolved into its 

 chemical elements, and if these be brought into the gaseous state, 

 will, at the temperature and pressure of the atmosphere, expand 

 about 1000 times. Hence the foregoing speck, if thus resolved 

 into gas, would occupy about ten cubic microns. But this volume 

 of gas at that temperature and pressure contains about a uno-ten 

 (10,000,000,000) of molecules, which for the most part will consist 

 each of two chemical atoms. Hence the number of chemical atoms 

 in our speck may be taken to be about two uno-tens. Our speck, 

 perhaps, consists of very complex organic molecules ; but however 

 complex each of these may be their number must nevertheless be 

 very great. For, let us make the liberal allowance of 2000 

 chemical atoms for each organic molecule, and the number of these 

 very complex molecules will be about ten millions. This is an 

 army quite large enough to admit of an immense amount of dif- 

 ferentiation within its ranks — of very active operations within and 

 among the complex molecules or between brigades of them — all of 

 which are ultra- visible events. These are facts which every biolo- 

 gist should keep constantly before his mind when carrying out his 

 investigations and interpreting them, and especially when he is 

 tempted either to speak or think of ' undifferentiated protoplasm.' 



4. A still more striking instance is presented when we consider 

 the operations of the human mind. Here I will make the usual 

 assumption, that every perception or other thought in the mind is 

 accompanied by a physical event occurring in the brain, which is 

 connected with it in such a way that neither presents itself without 



