A. AIR MAIL Balliol College

Oxford, England

May 4, 1948

Professor J. H. Sturdivant

Gates Laboratory

California institute of Technology

Pasadena 4, California

Dear Holmes:

I enclose some material about Unicam instruments, with the suggestion that you order an apparatus immediately, in case that one of the Unicam instruments has not already been ordered. I think that it could be paid for from any suitable fund, including the Rockefeller Fund.

It might well be that two of the instruments should be ordered, such as the single crystal goniometer and the two crystal goniometer.

I feel that it is important that we increase the facilities for x-ray photography at the institute. I was very much interested to see what they have in the x-ray lab at the Cavendish Laboratory, where H. B. Taylor is in charge. They have in one room, about 50% larger than our x-ray lab, a number of transformers and x-ray tubes, with associated apparatus. The total number of "windows" in this lab is 25—-most of the tubes have two windows, one on each of two sides, and in general they seem to operate two tubes from the same transformer. There is a bank of six transformers with controls down the center of the room, with an x-ray tube on each of two sides of each transformer, and an apparatus on each side of each x-ray tube. This gives 24 windows—I don't know where the 25th is. In addition they have a teaching laboratory with four windows, which is used only one term in the year for teaching, and is available for research the rest of the time. They use the multiple film technique for intensities, and seem to have some sort of photometric method of obtaining intensities, without, however, doing much in the way of integrating over the spot.

I must say that I think that there is one advantage to the English system, which is that research men are supposed to devote all of their time to research, rather than only half time or less. They do attend a few lectures, but not very many. It seems to me that perhaps we should revise our system somewhat, and have our graduate students take class work completely during the first year, and then practically 100% research during the other two years. The disadvantage to this is, of course, that we do not find out whether the man has any research ability during his first year.

Prof. Sturdivant May 4, 1948

They have been doing some interesting jobs at Cambridge. Two of the pyrimidine structures, that I saw last summer under way, are described in the first issue of Acta Crystallographica. They are finishing up a purine now. Corey would be interested to know that this work on pyrimidines and purines is designed as a start in an attack on nucleic acids. Also a girl there, Mrs. Douglas, has just finished determining the structure of a complicated intermetallic compound, Co2Al9, which has monoclinic symmetry. There are a number of other representatives of this structure. She is also beginning work on MnAl6. Their general program is to tackle the compounds of aluminum and the iron-group transition elements. In addition they have made some studies of the sigma-phase of chromium and iron, containing 50% of each of these two elements. This phase is a complicated powder diagram, which is, however, closely related to the body-centered structure. Instead of the successive lines of the body-centered structure, there is, in the general neighborhood of each of these lines, a complex of a half-dozen or more lines. They have not yet found out what the unit of structure is for this sigma phase. The single crystals of Co2Al9 and MnAl6, that they use are made by an interesting method, of annealing the an alloy with an excess of aluminum or cobalt--I am not sure which--and then subjecting the sample to electrolytic etching until the phase that is the more readily attacked is dissolved away, leaving a pile of crystals of the other phase. These crystals are perhaps two- or three- or four-tenths of a millimeter in two directions, and several millimeters along the needle axis.

There are three men working with Taylor on the structure of feldspar. They find, for example, that the first feldspars give rotation photographs corresponding to the normal feldspar structure, but with faint extra layer lines in between the strong layer lines. These faint layer lines indicate a unit either five times or seven times longer in the direction of the axis of rotation. Moreover, the two extra layer lines that are nearest to half-way between the main layer lines are strong, the others being very weak. This indicates an approximate doubling of the c axis. The explanation presumably is that the two kinds of feldspar structure, say a and b, tend to repeat, but not exactly, the sequences being ababa ababa or abababa abababa. It is clear that many interesting things remain to be done in this field. The difficulty in this feldspar work is that it is almost impossible to obtain single crystals of the micro-type feldspars that they are studying, the ordinary apparent single crystals being complex twins. The girl who has been making these photographs has taken an apparent single crystal, made x-ray photographs of it (Laue photographs), chipped off a piece, repeated the x-ray photographs, and continued until she obtained a small piece that gave a simple Laue diagram.

Prof. Sturdivant May 4, 1948

The piece that she obtained was about 0.02 X 0.02 X 0.01 millimeters, and yet they have used it to get first-rate rotation and Weissenberg photographs.

I am looking forward to getting back, and to talking with you about the x-ray work. I hope that you aren't working too hard now.

Sincerely yours,

Linus Pauling:par

enclosures