[258] See the very curious experiments of C. Matteucci. Traduit et extrait du “_Cimento_.”--Archives des Sciences Physiques et Naturelles; _Quelques Expériences sur la Respiration des Plantes_. Nov. 1846. [259] Consult _Rural Economy_, by J. B. Boussingault; _The Chemical and Physiological Balance of Organic Nature_, by Dumas and Boussingault; and _Agricultural Chemistry_, by Liebig. [260] The practical value of the discovery now described, will be best understood from the following letter from Mr. Lawson, of Edinburgh:-- Edinburgh, 1, George the Fourth’s Bridge, Sept. 8, 1853. MY DEAR SIR,--I am favoured with yours of the 5th, relative to my practical experience in the effect of the chemical agency of coloured media on the germination of seeds and the growth of plants. I must first explain that it is our practice to test the germinating powers of all seeds which come into our warehouses before we send them out for sale; and, of course, it is an object to discover, with as little delay as possible, the extent that the vital principle is active, as the value comes to be depreciated in the ratio it is found to be dormant. For instance, if we sow 100 seeds of any sort, and the whole germinate, the seed will be the highest current value; but if only 90 germinate, its value is 10 per cent. less; if 80, then its value falls 20 per cent. I merely give this detail to show the practical value of this test, and the influence it exerts on the fluctuation of prices. Our usual plan formerly was to sow the seeds to be tested in a hot-bed or frame, and then watch the progress and note the results. It was usually from eight to fourteen days before we were in a condition to decide on the commercial value of the seed under trial. My attention was, however, directed to your excellent work, “On the Physical Phenomena of Nature,” about five years ago, and I resolved to put your theory to a practical test. I accordingly had a case made, the sides of which were formed of glass coloured blue or indigo, which case I attached to a small gas stove for engendering heat; in the case shelves were fixed in the inside, on which were placed small pots wherein the seeds to be tested were sown. The results were all that could be looked for: the seeds freely germinated in from two to five days only, instead of from eight to fourteen days as before. I have not carried our experiments beyond the germination of seeds, so that I cannot afford practical information as to the effect of other rays on the after culture of plants. I have, however, made some trials with the yellow ray in preventing the germination of seeds, which have been successful; and I have always found the violet ray prejudicial to the growth of the plant after germination.--I remain, my dear Sir, Very faithfully yours, CHARLES LAWSON. CHAPTER XV. PHENOMENA OF ANIMAL LIFE.