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January 6, 1853. 

 WILLIAM HOPKINS, Esq., V.P., in the Chair. 



A paper was read, entitled " On Molecular Influences. Sect. 1. 

 Transmission of Heat through Organic Structures." By John 

 Tyndall, F.R.S. Received Oct. 20, 1852. 



In this paper the author has examined the influence exerted by 

 the molecular structure of wood upon the passage of heat through 

 the substance. Finding the usual modes of determining the con- 

 ductivity of bodies inadequate to his purpose, he has been led to 

 the construction of a new instrument which is capable of indicating 

 very slight diflferences of transmissive power. 



A cubical space is cut out of the centre of a rectangular slab of 

 mahogany. The same slab holds a thermo-electric pair of bismuth 

 and antimony, which are fixed in trenches cut out to receive them. 

 The junction of the pair (which is of a V-shape) abuts upon one of 

 the faces of the cubical space just mentioned ; the end of a wooden 

 slider forms the opposite boundary of the cubical space, and against 

 this end a platinum wire, bent several times up and down so as to 

 form a kind of micrometer-grating, is laid and imbedded in the 

 wood. A small projection of ivory abuts at each side of the bismuth 

 and antimony junction, and from one projection to the other a thin 

 membrane is drawn, thus enclosing a space in front of the junction, 

 which is filled with mercury. Two similar projections jut at the 

 sides of the micrometer-grating, and across from one projection to 

 the other, a second membrane is stretched, thus enclosing another 

 chamber in front of the wire. This chamber is also filled with mer- 

 cury, and against the wire a thin plate of mica is cemented, thus 

 preventing all contact between the two metals. From the free ends 

 of the bismuth and antimony bars wires proceed to a delicate galva- 

 nometer. 



The substances to be examined by this instrument are reduced to 

 the cubical form and placed between the two membranes ; the slider 

 being brought closely up against the cube, the latter is clasped 

 firmly between the rigid projections before-named. The membranes 

 are pressed gently against the two opposite faces of the cube by the 

 mercury behind, and thus a contact is secured which, as the mer- 

 cury is not changed during an entire series of experiments, remains 

 perfectly constant. This is a most important point in experiments 

 of this nature, for when the conditions of contact vary in even a 

 slight degree, comparable results are out of the question. This 

 remark of course applies exclusively to an inquiry like the present, 

 where the object is to detect minute diff'erences of molecular action. 

 The protruding ends of the micrometer- wire are united to the poles 

 of a small galvanic battery, and the wire is heated by the passage of 

 the current ; the heat is transmitted through the film of mica to the 

 mass of mercury in front, which thus becomes the source of heat 

 immediately applied to the face of the cube. The current is per- 

 mitted to circulate through the bent wire for 60 seconds. During 

 this time the heat passes from the face of the cube in contact with 



