D O C . 3 0 5 F E B R U A R Y 1 9 2 0 2 5 5 a few words to you: that I am pleased with all my heart that the idea of inviting you to come to Leyden as Extraordinary Professor has proved fruitful and has given us the magnificent assurance that through this formal confirmation your relations with us will expand and deepen, which will be of benefit to the very core of our science in Leyden and Holland. In my imagination I can already see you at our university’s venerable rostrum that was born of the struggle for freedom of conscience, smil- ing down at us and telling us about your communion with the gods and about the fine interplay of harmony by which hints of Nature’s laws are revealed, your kind eyes still sparkling with delight! This inaugural ceremony seems to me a fine sym- bol of how you will exert a lasting influence on scientific life here. As to the ways and means by which you wish to do so, you are as free as an air- borne bird. The offer leaves it entirely to you to arrange everything freely yourself, in accordance with relevant circumstances. Thus, best conditions are made for stimulating investigations, guiding ongoing analyses onto better paths, as well as exchanging fruitful ideas of every kind. Thus, with your Leyden professorship I also cherish the finest hopes for a flowering of the cryogenic laboratory. Virtually no one is so closely affiliated to it than you are. Many of the investigations per- formed there regard phenomena whose relevance to quantum theory you have rec- ognized and for whose analysis this laboratory is [somewhat of] an international in- stitution, insofar as the area of low temperatures is concerned. So your help can bring about much that is of benefit. You will perhaps find me very egoistical if I already immediately ask you to make available to me some of your precious time for devising strategies and iden- tifying problems. But I take that risk, dear friend! And I assure you that I find it just as great a fortune for the Cryogenic Laboratory as for theoretical physics that you will be connected with Leyden as one of our own. With a hearty handshake, I am very devotedly yours, H. Kamerlingh-Onnes. 305. To Edouard Guillaume [Berlin,] 9 February 1920 Dear Guillaume, I was delighted to receive your letter, because there is always a certain plea- sure attached to reviving older reminiscences by us ancient fellows. But your observations are definitely a bit foul and are based on an insufficiently sharp dis- tinction between the cases to which the presented equations refer (in your letter).