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permeates everything. All science seems worked with reference to it. Tyndall not only assumed it, but it was the foundation of his philosophy. While I was with him the other day Spencer started the point of using the term persistence of force rather than conservation. They had quite a spurt over it. But to-day Huxley used the term persistence of force. The experiments last night were first, testing oxygen and hydrogen separately; second, exploding them together; third, bursting iron bottles by freezing; fourth, exhibiting the formation of crystals by the electric light in a vacuum; fifth, formation of an immense spectrum on a screen, absorption of its different parts by colored glasses; and sixth, regelation of iron. He had splendid diagrams of the glaciers, but hardly referred to them. He was not still a moment, but bending and twisting in all possible shapes as if he had the St, Vitus dance—twisting his legs together, bending down to the desk, and working and jerking himself in all possible directions. Everybody was kept awake, entertained, and instructed. It was a work of enthusiasm.

One of the consequences of that first interview with Tyndall appears in the following extract from a letter of my brother's written to Mr. Spencer in March, 1863. He says: "I received the advance sheets of Prof. Tyndall's book on Heat, and beg of you to express to him my sincere thanks for the kindness. The Appletons will issue it at the earliest moment, the cuts being already nearly all re-engraved. It is a very fascinating and altogether remarkable book, and it will be a pure pleasure for me to work for its circulation. It can not fail to have a good sale."

A letter of Prof. Tyndall's to my brother relating to the publication of his work on Heat, and bearing date April 29, 1863, is the earliest one in my possession. It is as follows:

My Dear Sir: As soon as I received the letter with which you kindly favored me some months ago, I communicated at once with Mr. Longman and requested him to forward you the separate sheets of my work on Heat according as they appeared. I intended to accompany the sheets with a letter which should express my desire to leave the management with the Messrs. Appleton entirely in your hands, but I have been so knocked about—sometimes so ill, sometimes so hard worked, and sometimes engaged so far away from London—that I have delayed thus far to write you. My friend Spencer called to see me a few days ago, and from him I had the great gratification of learning that the book has interested you. Indeed, he read portions of letters from Mrs. Youmans and yourself which gave me very great pleasure. Since the appearance of the work I have had communications from many of my eminent Continental friends regarding it, and they, I am happy to say, concur in your opinion. A French translation of it has already been commenced. I can assure you that I have spared no labor to render a difficult subject intelligible, and it gives me great pleasure to find that I have, at least in some measure, succeeded.

I am now giving a short course of lectures on Sound at the Royal Institution. If I have time I may throw them into a readable form. I have for some time entertained the idea of publishing my lectures gradually and of afterward collecting them and fusing them into a book on general physics. But the time necessary to the proper accomplishment of the task deters me almost from undertaking it. However, it may perhaps be executed by slow degrees.