St Leonard's College
St Andrews
Feby 8th 1848
Dear Sir,
I cannot find a single crystal with a perfect centre, tho' I have examined thousands. I believe the reason of this is that minute crystalline plates are first deposited, and round one or two or three or sometimes four of these, the circular crystals are formed. This is certainly the case in many of the crystals. – Hence the crystals decompose first in the Centre, & after the central portion is gone, then the crystal has the appearance of being regular, or rather is regular, two or three fine black rings appearing around the vacant centre.
I have got very splendid crystals in which there is a thing like a bridge mn across the Centre, thus [diagram] the bridge being longer in dissolving than the parts beside it.
I am glad to be able to tell you how to preserve the Borax & Phosphoric Acid crystals, <1> forever. I make a plate of them, & then separate by force the two glass plates, on each of which the circular crystals will be found, some broken, others perfect, & others curiously split. I then pour upon each plate warm Canada balsam, <2> & thus preserve them perfectly embalmed. – I think the same end may be attained by keeping out the air with a wax rim. I have tried this, but time is necessary to shew its efficacy.
Fancying that circular seeds, might exhibit circular crystallisation, I placed the seeds of the Lycopodium <3> in Canada balsam. Their size is about the 1/940th of an inch, & among them are found numerous circular discs which exhibit the precise phenomena of the smaller circular crystals, namely a short black cross of 4 sectors of light with negative double refraction! The seeds or rather the discs among the seeds are composed of crystalline elements grouped round a centre, for we cannot suppose that the phenomenon is one of tension as in circular plates of glass slowly cooled.
If you wish to give permanence to your circular crystals after deliquescence has begun, separate the plates, & remove the fluid by blotting paper and heat. Pour on each face Canada balsam & cover them all with plates of Glass. In this way a curious collection of beautiful phenomena may be made, & permanently preserved. In some of my specimens the decomposition of the circular crystals is accompanied with the formation of large needles or bunches of prismatic crystals like sheaves of straw, grouped irregularly round a centre, which are themselves beautiful objects.
Since writing the above I have split a greatly decomposed crop of Circr crystals dried them, and covered them with Canada balsam. The effect is curious – Independent of having a duplicate, I have cleaved as it were individual crystals, and the separate halves are marked with complete circles as if drawn with ink, all concentric, and forming the circumference of circular discs of difft diameter. These black circles, whh are exceedingly sharp, appear also upon the bunches of prismatic crystals in which there is no black cross owing to the irregularity of their crystallisation.
Believe me to be Ever Most Truly yrs
D Brewster
H.F. Talbot Esqr
&c &c &c
Notes:
1. See Doc. No: 03677.
2. Club moss.
3. A translucent substance used as a cement in microscopy as it has a constant refractive index.