PROFESSOR IN BERLIN 347 



On June 20, 1883, Helmholtz writes to his wife, who had 

 gone to Paris in the middle of May for the funeral of her uncle 

 Julius von Mohl : 



I Geheimrath Herzog was here yesterday, and brought me 

 an invitation to join a sixty-seven days* journey to the Pacific 

 and back, from August 15 to October 22, for the Opening of 

 the Northern Pacific Railway, as the Company's guest ; thirty 

 distinguished men are to be invited from Germany, and they 

 say Count Lerchenfeld, the Minister Kruger, Georg Bunsen, 

 Gneist, and the Reichstags- President von Levetzow, are going. 

 Herzog promises princely accommodation on the journey, and 

 receptions. If one is to see America in this life this would 

 perhaps be the best opportunity imaginable. For this reason 

 I have not yet declined, although there are many obstacles in 

 the way, and it is really not essential that one should see 

 America, at any rate not for what I have to do in the world/ 



His wife did not approve of his undergoing the fatigues 

 inseparable from this journey; accordingly he declined the 

 invitation. 



I 1 have no particular wish to make a journey at present/ 

 he writes to her again on August i, while she was still detained 

 at an English watering-place by the protracted illness of their 

 son Robert. 'Just now I have some interesting experiments 

 on hand, that are beginning to go well, and have received my 

 new magnetic balance admirably made. But I notice signs 

 that I am getting worked out, so it can't go on much longer, 

 and the climate of Pontresina admits of no delay.' 



During this time Helmholtz had great pleasure in carrying 

 on a scientific correspondence with his son Robert, who was 

 pursuing his studies in chemistry, physics, and mathematics with 

 the utmost zeal. On October 20 he writes from Rome : 



'As to your experimental inquiries, I should recommend 

 you to find out if electrified air gives a double layer at the 

 surface of a conductor. Take a Kohlrausch condenser with 

 carefully cleaned plates, and test the tension between them. 

 Then charge one of them temporarily with an electrical 

 machine, and discharge it by a small flame that gives no 

 deposit of moisture ; then bring it back to the condenser, and 

 see if the difference of potential remains unaltered. Then 

 do the same with the opposite electricity. The experiments 



