On the Molecular Constitution of Crystals. 109 



another, but it was totally erroueous in theory, as Dr. WoUaston 

 has ably shown. 



Immediately after him, Dr. Wollaston published in the Philo- 

 sophical Transactions a theory of the manner in which the 

 common octahedron and tetrahedron may be built up of spheres 

 having a mutual attraction for each other. He evidently con- 

 sidered his ideas the same as those of Hooke, as he says he found 

 himself anticipated by that writer. He does not seem to have 

 prosecuted his inquiries very fai', indeed he leaves totally un- 

 noticed the innumerable other forms to be met with in nature. 

 He does, however, make some observations with regard to the 

 cube, but he does not appear to have satisfied himself; his words 

 are to this effect : — " If a number of white and an equal number 

 of black balls are so arranged that each white ball shall be equi- 

 distant from the four nearest black balls, and also from the 

 adjacent white balls, the form which they would take would be 

 a cube." 



I shall now endeavour to show that this theory of the forma- 

 tion of the octahedron is faulty, inasmuch as the molecules, if 

 left to themselves, would never assume such an arrangement. 



If any four spherical atoms become attached in consequence 

 of their mutual attraction, they will combine in the manner 

 shown in Plate I. fig. 1, and any fifth atom will attach itself, as 

 shown in fig. 2 ; but if we examine fig. 3, which represents the 

 arrangement which he supposes them to have, we will find that 

 any five adjacent spheres occupy a position such as shown in 

 fig. 4, a position which they evidently never would naturally 

 assume. 



Wollaston himself seems to have some lurking doubts of the 

 validity of this assumption, as he begins by showing how the 

 spheres will become aggregated if on a plane, and then from this 

 basis builds up the octahedron ; this, however, is the very man- 

 ner in which crystals do not form. 



The first who really investigated this most interesting subject 

 was the Abbe Haiiy ; his work forms an sera in the history of 

 the science. He perceived that many crystals were liable to 

 cleavage, and that thus in many cases new solids were obtained ; 

 he was led from this to consider, that, if this cleavage were con- 

 tinued long enough, he would ultimately arrive at the absolute 

 atom ; and he assumed, without sufficient foundation, that this 

 atom should have the same form as the solid obtained by cleavage. 



The forms which he considered the molecules to have were the 

 tetrahedron, paralleloj)ipcd, and the three-sided prism. He 

 clearly showed that many of the fcjrms could be constructed with 

 these elements, and he pointed out how some of the secondary 

 forms would result from certain decrements or omissions in the 



