From Horace Darwin   17 August 1874

Dear Mother

Will you tell Father that I have hunted in the only two book’s about the steam engine wh. I know of and can find nothing of any good. I had a talk with the Manager Mr Moberly, & he said that feed water heaters did not get so much choked up as the boiler, but I do’nt think this shews so very much, because the only kind of deposit wh. is much harm, I believe, is a solid insoluable crust wh. coveres the inside of the boiler; and the feed water heater may precipitate the carbonate of lime, as powder and not as “scale” as it is called; and if it does the precipitate would go into the boiler and might get turnde into the scale there.¹ What makes me think that this is possible is that there is a patent for preventing the scale by sending an electrical current from the water to the metal of the boiler, but whether this acts I do’nt know, we are trying it at the works but the boiler has not been cleaned out since it was put up, so they dont know how it works; if it does answer it shows that there is something very odd about it, so one can’t tell for certain. Mr. Moberly also said that he thought that 100^o Far.² would precipitate very little of the Car. of lime; I will get another talk with him. All he said only applied to fresh warter heaters. I looked at Watts Dictionary of Chemistry & found a good deal about it under. Water-Solution— gaves absorption of—Carbonate of Calcium—Carbonic anhydride— water does disolve less Carbonic acid the hotter it gets.³

I want to hear very much what answer Murray gives.⁴

Yours affec son | H Darwin