230 HERMANN VON HELMHOLTZ 



The explanation which Helmholtz gave in his popular lecture, 

 ' Ice and Glaciers/ of that mysterious and misinterpreted pheno- 

 menon the Fohn is very interesting, and the foundation of the 

 whole theory of rainfall. When the warm air of the Medi- 

 terranean is driven northwards by the south wind, a portion of it 

 is compelled to ascend the great mountain wall of the Alps. In 

 consequence of the diminished pressure of the air it expands by 

 about half its volume, is considerably cooled in temperature, and 

 at the same time deposits the best part of its moisture as 

 snow or rain. When the same air afterwards descends to the 

 valleys and plains on the north side of the mountains as the 

 Fohn wind, it is again condensed and grows warmer: thus 

 the same current of air that is warm in the plains on either side 

 of the mountains can be bitingly cold upon the heights, and 

 deposit snow there, while it is insufferably hot in the plains. 



The year 1865 brought a change in Helmholtz's domestic 

 relations; his mother-in-law, Frau von Velten, took up her 

 permanent abode at Dahlem, and during the long period that 

 elapsed before her death in 1881, only once returned to Heidel- 

 berg, when, in 1874, she came to stay with her married grand- 

 daughter Kathe. 



In the autumn vacation Helmholtz went as usual to Switzer- 

 land, where long and arduous excursions refreshed him in mind 

 and body: but he was soon recalled to Heidelberg by disquieting 

 accounts of the state of his son Robert. His wife again en- 

 deavoured to keep him away from home as long as possible : 



1 Enjoy your journey thoroughly, and get your poor head 

 well, dear Hermann, so that we may both be fresh and vigorous, 

 if we are threatened with new illness. We must keep our 

 courage up, if we are to pull through. Don't imagine that 

 I am giving way ; I am trying to keep well and cheerful, and 

 scold myself for my faint-heartedness, when I think of you and 

 your hatred of all exaggeration. . . .' 



But after the diagnosis of the physicians Helmholtz could 

 cherish no further illusions as to the nature of his child's illness, 

 and returned direct from Geneva, stopping only a few hours in 

 Freiburg, to listen once more to the strains of the organ which 

 he had admired so much in bygone years : 



'The organ is truly wonderful, from the point of view of 

 acoustics even more than from that of music. I confess that 



