To Rudolf Clausius   15th Nov. 1857.

15^th Nov. 1857.

My dear Clausius

The thought of writing to you occurred to me some time ago on reading your article ‘on the motion which we call heat’¹ in which I felt great interest. It was translated by our friend Hirst for the Philosophical Magazine, and I hope your paper on the ‘Electrolytes’² will soon appear also. I spent 6 weeks of the present summer beside the Mer de Glace³ and was from 10 to 12 hours daily upon the ice. On the 13^th of August, accompanied by Hirst and a single guide⁴ I repeated Saussure’s experiment of firing a shot on the summit of Mont Blanc.⁵ I was regarded by the authorities of Chaumouni⁶ as an exceptional case and was permitted to go where I pleased, and with as many & as few guides as I thought fit. Of course a considerable number of observations were collected during those 6 weeks, and I am now engaged in writing them out and discussing them. I dare say you saw a little paper of Huxley’s in the Philosophical Magazine⁷ some time ago. I have seen a good deal of the water and air-bubbles in ice since; the question is a very curious one for these water drops which are associated with the air-bubbles were assuredly once ice, and then the question arises how have they been melted? I think they have been melted by the quantity of heat which has been able to pass through the ice by conduction. The statement may appear paradoxical that ice can be melted by heat which has passed through another portion of the substance without melting it. But I think theory would lead us to expect a result of this kind. A particle at the surface of a mass of ice is in a very different mechanical condition from that of a particle within the mass, where it is surrounded and controlled on all sides by other particles. The quantity of heat necessary to give a particle at the surface the liberty of liquefaction, if I may use the term, would not I think be sufficient to give a particle within the mass the same liberty. But supposing a cavity—an airbubble, for example,—to exist within the ice, the particles surrounding the bubble would be in the same condition as those at the surface, and a quantity of heat may pass through the solid without melting it, which is capable of liberating the particles bounding the air cavity. I really see no other way of explaining the fact.⁸ One may perhaps form a rough conception of the thing by thinking of the transmission of motion through a row of elastic balls, the last one of the row being liberated. The explanation given by Agassiz and the Schlagintweits,⁹ is I think without basis; you know they suppose the air to be warmed by the heat which has passed through the ice, the air thus melting its surrounding walls.

While in Geneva¹⁰ I thought of you, and made an excursion to the place where the Rhone enters the lake¹¹ to see the colour of its water: it was quite thick and muddy, but it was surprising how soon the suspended matter settled, the river losing its distinctive character and being absorbed in the general hue of the lake. Now the reason I thought of you was this. I am afraid we are too easily satisfied with the explanation usually given of the colour of the lake of Geneva; that it is merely the colour of the water itself. I know that pure water is blue, but the question still remains: is this blueness sufficient to account for the exceptional depth of hue of the lake of Geneva? Would a stratum of distilled water show an equal depth of colour? The river brings down an immense quantity of sediment, partly derived from the rubbing action of the glacier of the Rhone¹² upon its bed. Mud is extremely plentiful, and I think it must be the case that some of these mica¹³ plates are so small as to remain suspended in the lake throughout its entire course. Faraday has obtained suspended gold so fine that it takes months to sink to the bottom of a bottle 6 inches high. But if the plates be in the lake, they must have an action like that which you attribute to the vesicles of water in the air, and hence, to their presence the colour of the water may be in some measure due. Do you not think this possible? I find the colour of snow depends materially upon its state of aggregation: on the summit of the Stelvio¹⁴ last year where fine new snow was lying the hole made by my alpenstock¹⁵ was filled with a delicate blue light. lower down the mountain, while the snow was more coarsely granular, this Effect almost wholly disappeared. I noticed the same upon the top of Mont Blanc this year: a hole made by the iron of my alpenstock was sufficiently deep to cause the colour to show itself in great beauty. You once mentioned to me I think that Brücke had made some experiments with the vapour of sulphur: can you tell me where they are described?¹⁶ and tell me at the same time whether you retain your former opinion regarding the colour of the sky.

Have you formed any conception as to the manner in which a transverse undulatory motion, like that of radiant heat can build a tree in opposition to the force of gravity:¹⁷ or how the chemical rays produce the tensions of vegetable & animal tissues? I was going to ask a few more questions, but I have given you enough of trouble for the present.

Believe me dear Clausius | Most sincerely yours | John Tyndall

RI MS JT/1/T/173