Dr. Tyndall on Molecular Influences. 121 



5. Tinstone Pseudomorphs. — The well-known pseudomorphs 

 of tinstone from Cornwall — after feldspar twins, flattened parallel 

 to the side planes L, or (x Poo ) of Naumann — consist almost 

 invariably of a granular mixture of cassiterite, quartz, and de- 

 composed feldspar. Two specimens showed respectively, the 

 following specific gravities: 5*313 and 5*68. The first, in a 

 blowpipe assay, yielded 43*6 per cent, of metallic tin, which is 

 equivalent to a per-centage of 55*46 of the binoxide. In the 

 centre of this specimen two little crystals of pure cassiterite were 

 met with, proving the more or less chemical origin of the pseu- 

 domorphs. For their mode of production, we must look pro- 

 bably to the action of chlorides in solution or sublimation. 



XVIII. On Molecular Influences. — Part I. Transmission of Heat 

 through Organic Structures. By John Tyndall, F.R.S.* 



THE various solid substances which are met with in nature 

 allow themselves to be classed under three general heads :—r- 

 Amorphous, Crystalline and Organized. In amorphous bodies 

 the component particles are confusedly mingled, without any 

 regard to symmetry of arrangement. In crystalline bodies, on 

 the contrary, the particles are symmetrically arranged ; the mass 

 appears as if built up according to certain architectural rules, 

 and the result is an exterior form whose angular dimensions are 

 perfectly constant for all crystals of the same class. Organized 

 bodies, as the name implies, are bodies endowed with, or com- 

 posed of, organs formed with reference to the special functions 

 they are intended to discharge, and in the construction of which 

 a molecular architecture of a very composite order comes into 

 play. The granules, cells, glands, tubes, &c. of animal and 

 vegetable tissues are all, of course, the visible products of this 

 architecture. Crystalline bodies appear to bridge the chasm 

 which separates the amorphous from the organized. Like the 

 former, they are devoid of the powers of assimilation and repro- 

 duction — like the latter, their particles are arranged according 

 to rule ; as if nature, in the case of crystals, had made her first 

 structural effort. The student of nature has ever looked upon 

 these molecular combinations with an inquiring eye, and, per- 

 haps, at no age of the world more than at present. The mole- 

 cular peculiarities of any substance declare themselves by the 

 manner in which a force is modified in its passage through the 

 substance. The polarization and bifurcation of a luminous ray 

 in doubly refracting media is an old example of molecular action ; 

 and the rotation of the plane of polarization, observed by Professor 

 Faraday, may be the result of a mechanical change of the me- 



* From the Philosophical Transactions for 1853, part ii. ; having been 

 received by the Royal Society October 20, 1852, and read January 6, 1853. 



