454 Addiimial Notes. 



times sliding backwards and forwards, make the other ones 

 much more magactical than themselves, which are then to 

 be taken up and used to touch the former, which are in suc- 

 cession to be laid down horizontally in a line." — Botanic 

 Garden, Part I. Canto ii. p. 48, note. New- York edit. 



Dr. Knight's method of making artificial magnets, re- 

 ferred to by Dr. Darwin in the above-mendoned note, was 

 as follows: He reduced iron to a very subtle powder, made 

 it into a paste with oil, moulded the composition into pieces 

 of a convenient form, dried them before a moaerate fire, 

 and then imparted to them the magnetic virtue, by placing 

 them between the extreme ends of his large magazine of ar- 

 tificial magnets, for a few seconds or more, as he thought 

 requisite. 



After Michel, the manufactory of artificial magnets re- 

 ceived further improvements by Mr. Canton, in 1756, and 

 by M. Antheaume, in 1766. 



Magnetical Theory o/'^pinus. p. 35. 



The theory of this celebrated philosopher of St. Peters- 

 burgh may be comprised in the following propositions. 



1 . There exists a substance in all magnetic bodies, which 

 may be called the magnetic fluid ; tlie particles of which re- 

 pel each other with a force decreasing as the distances in- 

 crease. 



2. The particles of magnetic fluid attract and are attracted 

 by the particles of iron, with a force that varies according to 

 the same law. 



3. The particles of iron repel each other according to the 

 same law. 



4. The magnetic fluid moves, without any considerable 

 obstruction, through the pores of iron and soft steel; but it 

 is more and more obstructed in its motion as the steel is tem- 

 pered harder; and in hard tempered ste^l, and in the ores of 

 iron, it is moved with the greatest difficulty. 



5. When the quantity of this fluid contained in iron is 

 such that the accumulated attraction of a particle for all the 

 iron balances, or is equal to, the repulsion of all the fluid 

 which the iron contains, the quantity may be said to be the 

 natural quantity of the iron, which may then be said to be 

 in its natural slate. 



6. The magnetic fluid may be abstracted from one end of 



