Faraday to Jacob Herbert   8 May 1846

Royal Institution | 8 May 1846

My dear Sir

Immediately on the receipt of your letter¹ & the specimens of water from the Small's lighthouse I carefully examined the latter and should have sent the results to you before now had I not judged it better to obtain what information I could from Mr. Wilkins respecting the roof of the lighthouse that might serve to illustrate the nature of the case.

I received three bottles of water from the Smalls - the following are their lables [sic] & the results obtained with them[.]

No.1. "This bottle of water was taken from the Smalls. To be inspected by the Honorable Corporation; because we find that it contains Verdigris, by putting soap in it turns green. H. Bourne² Assistant light keeper". This water contains a very considerable proportion of copper not as verdigris but as muriate. It contains also Sulphates & muriates & especially Sea salt.

No.2. Water taken out of the water cask. Small's light[.] This water contains a very small trace of copper as a muriate but the proportion is so minute & unimportant that it required all my care to find it. There were also Sulphates & muriates of the alkalies & earths but not any thing like the proportion in the water No.1.

No.3. Water taken out of the Dripstone, Small's light. This water was quite pure as respects copper but contained the proportion of sulphates & muriates which is usual in ordinary good waters[.]

The water No.1 would be poisonous if used for food or drinking & is manifestly unfit for that or any but very ordinary purposes. No.3 water is very good & sweet & fit for all uses - but care should be taken (& I conclude from the Superintendents³ letter⁴ has been taken) that none of the water No 1 should run into it. Such arrangements should be made (& they must be very easy) as that by no forgetfulness or omission any water like No.1 should have access to the supply used for cooking & food. The water No.2 would be very good for food but for that minute trace of copper which is probably in it by accident; & it is very good for all household & ordinary purposes.

As to the cause of the condition of the water No.1 I cannot perceive how the use of the ventilation tubes can have any thing to do with it for it is impossible that the steam & lamp smoke which passes up them can carry copper with it to the outside though the water which drops from them on the inside may occasionally be cupreous. On a consideration of all the circumstances I conclude that the effect has been produced by the salt & spray of the sea. It is impossible for sea water to come into contact with copper especially in the air without acting upon it & corroding & dissolving the copper. If the wind were to drive a little spray on to a copper roof or cowl and then a shower of rain washed the surface the water would contain copper more or less according as the time of contact between the copper & salt had been longer or shorter - and I conclude that No. 3 is water produced in a case of this kind because of the Sea salt which I find in it[.]

Mr. Wilkins has written to the work man who was at the Small's & has received the following answer from him. "The light keepers at the Small's are supplied with water for domestic use by the Steamer from Milford taken out in casks. They also catch the rain from off the lanthorn & gallery (which is covered with lead) from thence conveyed by a pipe into a tub. There was no new copper fitted to the roof & all is painted"⁵. As the effect of Spray on lead (though not so readily as of copper) would also be to dissolve gradually traces of that metal I would beg to submit as a rule in every possible case that no water gathered from lead or copper roofs & galleries exposed to the spray of the sea be used for culinary purposes or as food - and that where it must be gathered for domestic purposes it be not received at the beginning of a shower but only in the middle or latter end of the period of rain⁶[.]

I am My dear Sir | Your Very faithful Servant | M. Faraday

Jacob Herbert Esq | &c &c &c