244 Professor TyndaVs Lecture on Force, 



sensation which we call heat. We, moreover, know the amount 

 of heat which a given amount of mechanical force can develope. 

 Our lead ball, for example, in falling to the earth generated a 

 quantity of heat sufficient to raise the temperature of its own mass 

 three-fifths of a Fahrenheit degree. It reached the earth with a 

 velocity of thirty-two feet a second, and forty times this velocity 

 would be a small one for a rifle bullet. Multiplying three-fifths by 

 the square of forty we find that the amount of heat developed by 

 collision with the target would, if wholly concentrated in the lead, 

 raise its temperature 960 degrees. This would be more than suf- 

 ficient to fuse the lead. In reality, however, the heat developed 

 is divided between the lead and the body against which it strikes ; 

 nevertheless, it would be worth while to pay attention to this 

 .point, and to ascertain whether rifle bullets do not, under some 

 circumstances, show signs of fusion. 



From the motion of sensible masses, by gravity and other means, 

 the speaker passed to the motion of atoms towards each other 

 by chemical affinity. A collodion balloon, filled with a mixture 

 of chlorine and hydrogen, was hung in the focus of a parabolic 

 mirror, and in the focus of a second mirror, twenty feet distant, 

 ^ strong electric light was suddenly generated. The instant the 

 light fell upon the balloon the atoms within it fell together with 

 explosion, and hydrochloric acid was the result. The burning of 

 charcoal in oxygen is an old experiment, but it has now a 

 significance beyond what it used to have ; we now regard the act 

 of combination on the part of the atoms of oxygen and coal, ex- 

 actly as we regard the clashing of a falling weight against the 

 earth. And the heat produced in both cases is referable to a 

 common cause. This glowing diamond, which burns in oxygen 

 as a star of white light, glows and burns in consequence of the 

 fallincr of the atoms of oxygen against it. And could we mea- 

 sure the velocity of the atoms when they clash, and could we 

 find their number and weight, multiplying the mass of each atom 

 by the square of its velocity, and adding all together, we should 

 get a number representing the exact amount of heat developed by 

 the union of the oxygen and carbon. 



Thus far we have regarded the heat developed by the clashing 

 ^f sensible masses and of atoms. Work is expended in giving 

 motion to these atoms or masses, and heat is developed. But we 

 reverse this process daily, and by the expenditure of heat execute 

 work. We can raise a weight by heat ; and in this agent we 



