Faraday to the Editor of the Times   22 September 1859

Sir, - The Trinity-house, in its care for the health of the people engaged under it in the superintendence of lighthouses, has at different times sent to me, as its scientific adviser, certain specimens of waters, which were supposed to be injurious to the persons using them. Lighthouses are, of necessity, often placed in situations where water is obtained with difficulty, and they are frequently dependent more or less, upon that which is gathered from rain falling upon the leaden roofs, galleries, and gutters of the towers and cottages occupied by the keepers. Now, the salt of the sea spray, which often reaches these roofs, &c., even when they are half a mile or more from the shore, causes the rain water which falls upon them to dissolve a portion of the lead, which is larger or smaller under different circumstances, and at times rises up to a quantity injurious to health, and poisonous¹. The water thus contaminated by lead, or rather chloride of lead, is peculiar in this, that it does not lose the poisoning substance either by boiling or by exposure to air, for the metal remains soluble after one or both of these processes. I ascertained that if a little whiting, or pulverized chalk (carbonate of lime), were added to such water, and the whole shaken or stirred together, the lead immediately assumed the insoluble state; so that when the water was either filtered or left to settle the clear fluid was obtained in a perfectly pure and salubrious condition. The process of purification is, therefore, exceedingly simple, for if some powdered chalk or whiting is put into the cistern in which such rain water is collected, and stirred up occasionally after rain, the water may, with the greatest facility, be obtained in a perfectly fit state for all culinary and domestic purposes.

The Trinity-house has supplied this information to all the cases needing it which have come to its knowledge, but I find that some cases occur not under its charge, that there are others not connected with lighthouses, and others again in other countries, in all of which this piece of simple practical knowledge may be useful. Under these circumstances I have thought that you, Sir, would not refuse the service of that special and extensive power of publication and instruction which The Times possesses, but use it to carry this knowledge to the many dispersed persons who may greatly need and yet have no other means of obtaining it.

I am, Sir, your obliged and faithful servant, | M. Faraday

Royal Institution, Sept. 22.