A Reply to Mr. Donovan's Olservativns. 329 



Fig. 9. Root common to all rich earth: the root being 



larger still. 

 Fig. X. The first sort of pump ; the second sort of pump, 



fig. XI. 



LX. A Reply to Mr. Donovan's Observations, ^c. on. Mr. 

 De Luc's Paper published in our Number Jor February, 

 By J. A. De Luc, Esq. F.R.S. &fc. 



To Mr, Tilloch. 



SiRj — In your Journal for March, art. xxxvii. I find *' Obser- 

 vations orr a Paper by J. A. De Luc, Esq. containing some Re- 

 marks on Mr. Donovan's Reflections concerning the Inadequacy 

 of electrical Hypotheses ; by M. Donovan, Esq." 



If philosophical controversies were always carried on with such 

 a tranquillity and fairness as reigns in that between Mr. Dono- 

 van and myself, such discussions would better forward true na- 

 tural philosophy than they commonly do. I hope it will be the 

 case when I shall have explained myself respecting what appears 

 a disagreement between us. 



A first point relates to the object of excitation. I had not 

 present to my memory all the parts of Franklin's theory with re- 

 gard to that part of electric science, when finding it in Mr. Dono- 

 van's paper, and considering it as his own opinion, I made some 

 objections against it ; therefore he has reason to say that it does 

 not prove against /lim, tut against Franklin; in proof of which 

 he quotes the Doctor's work, " Experiments and Observations on 

 Electricity," published in London in 1749. 



The most important point ia (,ur controversy concerns Volta's 

 theory, of which I thought to give a proof by an experiment 

 against which Mr. Donovan objects. The theory of that justly 

 celebrated electrician is, that the standard of plus and idvuis in 

 our observations is no Jixed point, but changeable, heing; ihe ac- 

 tual electric state of the ambient air. My experiments to prove 

 it, which I still consider as decisive, Mr. Donovan accurately 

 describes, and I shall repeat them in his own words ; in which, 

 however, he uses an expression which has misled him, and which 

 J shall point out afterwards. 



** By the continual dispersion of electricity in a room (where 

 an electric machine was worked, having a poi7it fixed to the 

 prime conductor), the air of the room was rendered positive ; a 

 pair of insulated pith-balls in the natural state was brought in 

 from an adjoining room; they were diverged negatively; but when 

 ithey returned to the room whence they came, the natural state 



was 



