In the end of May, or beginning of June last, I happened to stand near a locomotive engine on the Greenwich Railway, which was discharging a vast quantity of high-pressure steam by its safety-valve. I chanced to look at the sun through the ascending column of vapour, and was struck by seeing it of a very deep orange-red colour, exactly similar to that of dense smoke, or the colour imparted to the sun when viewed through a common smoked glass.

I did not pay much attention to the fact at the moment, nor attempt to vary the experiment; but, reflecting on it afterwards, it seemed to me not only as in itself very singular, but as still more extraordinary, because I had never heard of a property of steam which must have been witnessed by thousands of persons. Some months after (in the end of October), being on the Newcastle and Carlisle Railway, I resolved to verify the fact, which I had no difficulty in doing, and I farther discovered a very important modification of it. For some feet or yards from the safety-valve at which the steam blows off, its colour for transmitted light is the deep orange-red I have described.