HI8T0EY OF THE ATOMIC THEORY. 147 



A note gives a fuller account of Dr. Cullen's views ; it was 

 written in the year 1759. Elective attractions were in reality 

 definitely laid down and presupposed in Geoffroy's tables; 

 but the investigation and elaboration was needed.* At pre- 

 sent we must consider Dr. Cullen as the first who used the 

 words and explanations in the manner afterwards made so 

 famous by Bergman. 



• Note E., p. 45., Culleo's Life, by Thomson, p. 570. Appendix — The 

 following passages from a letter, written by Dr. Cullen to his friend and former 

 pupil. Dr. George Fordyce, of London, in October, 1759, contains bis own 

 statement of his views with regard to double elective attractions. " I must 

 give you the manner of considering the subject, which I fell upon last session, 

 and shall continue to employ as the most easy and simple. I begin with your 

 third and fourth cases, and to these one general rule applies, viz., that when 

 two mixts (compounds) are applied to each other, if in each mixt there is a 

 substance, that from the table of elective attractions, is by itself capable of 

 decomposing the other mixt, the attractions between these substances and the 

 substances they attract in the opposite mixt, must always be greater than the 

 attractions subsisting in the mixts applied to each other ; and therefore, &c. 

 Thus, if nitrum argenti and common salt are applied to each other, as by the 

 table of elective attractions, the nitric acid in nitrum argenti, is by itself 

 capable of decomposing the other mixt, common salt ; and the muriatic acid in 

 common salt is capable of decomposing nitrum argenti: the attractions between 

 the nitric acid and the soda, with the attraction of the muriatic acid and the 

 silver must be always greater than the attractions subsisting in the mixts, 

 nitrum argenti and common salt, that were applied to each other. This I 

 illustrate by the diagram adjoined. Let there be two rods intersecting one 

 another, and moveable on a common axis at the point of intersection. At the 

 extremities of each let there be placed substances that have an attraction for 

 each of the substances on the extremities contiguous to them, and let the 

 attractions be expressed by the letters W, X, Y, Z. The rest of the illustra- 

 tion will readily appear from the diagrams. 



Nitric W SUver. Nitric W SUver 



Add. 



. Muriatic 

 Soda. X Acid. Mercury. X 



Y 7 X and Z 7 W by table. Y 7 X and Z 7 W by table. 



Ergo Y + Z 7 X + W Ergo Y + Z 7 X -h W 



You see that the prevailing attractions are here determined from the table 

 of single elective attractions. 



We are now come to the only difficulty in the affair of double elective 

 attractions in instance past. To this our general rule does not apply. 



