256 



The most general conceivable thermo-electric relations of a crys- 

 talline solid, or body possessing, inductively or structurally, different 

 physical properties in different directions, are next examined. It 

 is shown how a metallic structure may be actually made up of pieces 

 of different non-crystalline metals, which, taken on a large scale 

 compared with the dimensions of the heterogeneous elements of 

 which it is composed, will be found to exhibit the most general type 

 of thermo-electric directional relations indicated by the abstract in- 

 vestigation ; and it is inferred that it would be wrong to limit the 

 general expressions by any particular assumption, even if we only 

 discover simpler types of thermo-electric relations in natural crystals. 



The general equations determining the thermo-electric currents 

 in any naturally, inductively, or structurally crystalline solid ; result- 

 ing either from a completely specified distribution of temperature 

 through it ; or from given external appliances of heat, on which, and 

 on the thermo-electric currents themselves, the distribution of heat 

 through the interior will depend ; are investigated. 



Certain particular applications of the general equations are also 

 made ; and the thermo-electric properties of metallic structures (laid 

 before the Society as solids actually possessing the properties refer- 

 red to), are investigated. 



The paper in which this extension of the theory is described, 

 includes a more developed account of the theory of thermo-electric 

 currents in non-crystalline conductors, formerly communicated, than 

 has been hitherto printed ; with a simplification in the fundamental 

 equations introduced without hypothesis, by the adoption of a ther- 

 mometric assumption proposed as the foundation of an absolute scale 

 of temperature, in consequence of thermo-dynamic experiments on 

 air recently made by Mr Joule and the author. It also includes a 

 brief outline of some experimental investigations undertaken to an- 

 swer questions proposed in the former theoi'etical communication, 

 and suggested by various considerations which occurred in the 

 course of the research, and by the new part of the theory now com- 

 municated to the Royal Society. 



7. On the Structure of Diatomacea. By E. W. Dallas, Esq. 



The author directed attention to the following list of species, 

 which, although imperfect, exhibits great variety in the forms, 

 shewing the Medway to be very fertile in these organisms : 



