2 1 8 D O C U M E N T 2 0 1 F E B R U A R Y 1 9 2 6 but not . Let n be the number of quanta per unit volume, then ( ) is the den- sity of the energy current. This quantity is, owing to the symmetry of the energy tensor, equal to the momentum density, i.e., equal to pn. Therefore _____________________________ I would like to say two things about the quantum-theoretical calculation, which is very interesting. Your quantum condition[4] 1 should read 1. This I have found by reducing the problem to one variable with the coordinates (ϕ = rotation angle). Then, of course, the quantum condition reads . 1 This condition, recalculated for , yields the above formula. Secondly, follows clearly from experience with HCl vapor.[5] The ques- tion must therefore read: Is your formula (8) an expression of experience?[6] Kind regards, your A. Einstein 201. From Hendrik A. Lorentz Haarlem, 22 February 1926 Dear Colleague, Mrs. Kamerlingh Onnes has asked me to communicate to you the sad news that her husband passed away yesterday after an illness of a few days.[1] It started last Tuesday with apparently harmless bronchitis however, his situation deteriorated quickly and soon pneumonia with a high fever set in. I still saw him for a few mo- ments on Saturday and he also talked to me. Then, all hope was given up and the peaceful end came yesterday morning after several hours of unconsciousness. p E V1, ⋅ = p E V1 ----- -= EnV1 E hν = p EV1 = p1q1 p1q1 – p2q2 q2p2 –+ h 2πi -------- = p1q1…………………… h 4πi -------- = q* aeiϕ = p*q* q*p *– h 2πi -------- = q1, q2, p1, p2 α 2 -- -=1