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 strength of the cloth is greatly increased. It also becomes semi-transparent, and dyes far more rapidly than ordinary cloth, this being due to the swelling up of the cell-walls in the fibre. Owing to the expense of the treatment, the use of mercerised cloth has been hitherto limited to special applications, e.g. the manufacture of ‘calico-printers' blankets,’ in which increased strength of the fabric is required. In 1851 Mercer, who was one of the jurors of the International Exhibition held in London in that year, and therefore excluded from the ordinary distinctions, was awarded a council medal for the discovery of mercerisation. In 1852 he reluctantly assented to becoming a fellow of the Royal Society. In 1858 he contributed a paper to the meeting of the British Association at Leeds on the reducing action of light on persalts of iron, and their subsequent treatment with potassium ferricyanide, which yields a blue colour, varying in depth according to the intensity of light to which they have been exposed. The experiments were originally made by Mercer in 1828, and had been rediscovered by Robert Hunt [q. v.] The discovery of this photo-chemical action has given rise to many technical applications; Mercer himself proposed to utilise it for recording the intensity of sunlight, and Jordan has since practically carried out this suggestion in an instrument at present employed in meteorological observatories. At the Leeds meeting of the British Association Mercer also read a paper ‘On Relations among the Atomic Weights of the Elements;’ but he did not succeed in obtaining any results of importance in a field which has since proved fertile in discoveries. In 1859 his wife died, and from this time forward Mercer seems to have given up his scientific work. In 1861 he was placed on the commission of the peace for the county of Lancaster, but was judged by those who knew him to be too merciful for a magistrate (, Life, p. 266). In 1862 he served as a juror for the second international exhibition. A severe cold, brought on by falling into a water-reservoir in 1864, was the cause of a painful disease, of which Mercer died on 30 Nov. 1866. He left behind him two sons and two daughters.

In his private life Mercer was eminently unselfish and lovable. Endowed with the perseverance and business capacity necessary to raise himself from poverty to affluence, he was never grasping; and although he patented some of his inventions, he freely gave away many others, which brought large sums of money to those who profited by them. Through life he took an anxious interest in religion and religious affairs. In 1849 he seceded from the Wesleyans and returned to the established church, but, with characteristic liberality of mind, he continued to give material help to the local Wesleyan institutions. He was an ardent reformer, and was probably much influenced in his views by a short acquaintance with Richard Cobden [q. v.], who, with two partners, acted as the London agent for Messrs. Fort Bros. from 1828 till 1831 (, Life of Cobden, i. 15–18). In his experimental discoveries Mercer displayed great fertility of invention and a remarkable insight into chemistry. His classical researches on catalytic action, on the constitution of the ferrocyanides and of bleaching powder, and his anticipation of Pasteur's germ theory (communicated in a letter to Playfair), show the true scientific temper. There can be no doubt that had he devoted himself entirely to research he would have been among the most distinguished chemists of the day.

Among Mercer's more important discoveries, besides those already quoted, may be mentioned: (1) the use of potassium ferricyanide and potash for the discharge of indigo (Mem. Chem. Soc. iii. 320); (2) the use of arseniates as a substitute for phosphates in the process of ‘dunging;’ (3) the treatment of woollen fabrics (delaines, &c.) with a weak oxidising agent before printing; (4) the manufacture of sodium stannate and stannite in the dry way; (5) the production of ‘sulphated oil’ for the Turkey-red process; (6) the discovery of the solubility of cellulose in ammoniacal copper solutions. 

MERCER, WILLIAM (1605?–1675?), lieutenant-colonel and poet, was born probably at Methlic, Aberdeenshire, about 1605, his father, John Mercer, being at the time minister of that parish, and being afterwards translated to the church of Slains, where he officiated till his death in 1637. William was a wild youth, and running away from school, served as a soldier in Denmark and Sweden,