BEYOND THE LIMITS OF VISION. 9 



Sir William Thomson has made some very ingenious approxi- 

 mations to the actual size of molecules from such entirely inde- 

 pendent data as the following : First In the dispersion of light 

 in the spectrum how small must be the original particles in the 

 glass or the crystal of the prism, to retard light waves of 60,000 

 to the inch, which are the blue, and not much to retard those of 

 40.000 to the inch which are the red ; for this is the true sig- 

 nification of the dispersion of the colors in the solar spectrum. 

 Second How many molecules thick must be the film of a soap 

 bubble to resist the force and tension necessary to draw it out to 

 an extreme thinness. And : Third Knowing how much elec- 

 tricity or heat is developed by the application of thin plates of 

 different metals as copper and zinc, how thin would they have to 

 be to develop the same amount of heat which the same quantity 

 of these metals produces when they are alloyed to form brass ; 

 for then they are applied atom to atom. In each of these cases 

 his estimates of the probable size of molecules came to nearly 

 the same uniform result, and furthermore, to the same result 

 which is brought out by a far more accurate and reliable culcula- 

 tion, which I will more particularly describe. 



It is now a well established principle that the molecules of any 

 gas, as hydrogen, oxygen or the air we breathe, are in constant 

 and rapid motion one striking against another and bounding off 

 to hit a third, and so on continually, precisely as billiard balls 

 would act under the same impulse and without friction. This is 

 the mode of motion known as heat; and the hotter the gases 

 the swifter fly the particles. 



The hydrogen that fills a balloon resists an outside pressure of 

 fifteen pounds to the square inch ; and it does it by this incessant 

 bombardment of the atoms of the gas against the inside of the 

 containing bag. Now the weight of a given volume of hydrogen 

 being well known, it is only a matter of calculation to estimate 

 with what velocity its parts must be hurled against the sides of 

 any containing receptacle to resist such an outside pressure. It 

 is found that its particles must fly about in every direction with 

 the average velocity of 6,055 feet in a second, about 70 miles a 

 minute. The particles of the air, being seven and eight times 



