WCP2184

[1]

Science Schools

South Kensington S.W.

4^th March 1886.

Dear Mr. Wallace,

Your letter to Prof Meldola¹has been forwarded to me through Mr Symons.²

The specific gravity of the glass composing the Krakataõ ash is about 2.5 — that of the commoner pumice dust 2.3.

[2]

The size of the particles carried to the distance say of 1000 miles from the volcano varies greatly[.] What is of greater importance than size is form. How this pumice dust consists of wonderfully thin plates & threads of the glass — sometimes vesicular. The large area of these in proportion to their weight [3] facilates that adhesion to the air by which they are prevented from falling. It would be easy to measure the area of these particles but not their thickness — so that their absolute weight could be arrived at only with very great difficulty, if at all. They are like mica scales suspended in water –

I have specimens of different ashes &c. which I could supply you with, but I feel sure that it would be almost if not [4] quite impossible to determine the absolute weight of the particles.

It may interest you, as bearing on your present enquiry, that some time ago I received from the British Consul at Genoa, through the meteorological office, some dust brought from the African coast deserts during a violent ^{& long contained} storm. It included grains of quartz & other minerals having diameter of up to between ²/₁₀₀₀ & ³/₁₀₀₀ of an inch.

I shall be very happy to show you this in any other specimen of far travelled dust red rain &c in my possession.

Yours very truly | John W Judd³ [signature]