From H. C. Sorby   22 March 1876

My dear M^r. Darwin

I am sorry you should have felt any kind of hesitation in writing to ask my help in any of your researches since at any time I shall be only too glad to do what I can to assist in any experiments that you may think it desirable to make.¹ I only wish I could do more than I fear is possible in the case you name. Assuming that the sugar is glucose (i:e: grape sugar) the most delicate test I know is to take an ammoniacal solution of oxide of copper so dilute as to be only just a faint blue and to observe whether it is decolourized by boiling with the suspected substance. I really cannot say how small a quantity of glucose could be detected in this manner, but it must be a very small fraction of a grain—perhaps less than $\frac{1}{100}$th. Perhaps other tests might be found but it would require considerable research to be sure. Even this is not a certain proof but if no decolourization occurred it would prove that glucose is absent or in very minute quantity. I do not know any very delicate test for cane sugar.²

In order to use the test one ought to have the leaves tolerably fresh and damp. If you like to send some I will try what I can do but will not promise that the results will be satisfactory.

All being well I must leave here for London on April 4^th. so you ought to send the leaves as soon as you can.

I inclose a ticket for my conversazione. I should be delighted to see you at it but if you could not come, perhaps one of your sons could. I have therefore filled it up merely M^r. Darwin.³

Yours very truly | H. C. Sorby

I will send you a corrected copy of my address.⁴ Before you publish any thing on the subject, drop me a line, so that I may let you have the most recent results.