A considerable portion of the mixture is treated with boiling water and allowed to settle. Genuine coffee affords a clear and limpid infusion; many foreign grains yield a thick gummy liquor, resulting from the starchy and saccharine matters contained. An infusion of pure coffee, if treated with solution of cupric acetate and filtered, will show a greenish-yellow colour; if chicory be present, the filtrate will be reddish-brown. As a rule, samples of ground coffee which are much adulterated, pack together when subjected to a moderate pressure. Owing to the low density of a coffee infusion (due to its almost entire freedom from sugar), as compared with that of the infusions of most roots and grains, it has been suggested by Messrs. Graham, Stenhouse and Campbell, to apply the specific gravity determination of the infusion obtained from the suspected sample as a means for detecting adulteration. The results afforded are fairly approximate. The solution is prepared by boiling one part of the sample with ten parts of water and filtering. The following table gives the densities, at 15°·5, of various infusions made in this manner:-- Acorns 1·0073 Peas 1·0073 Mocha coffee 1·0080 Beans 1·0084 Java coffee 1·0087 Jamaica coffee 1·0087 Costa Rica coffee 1·0090 Ceylon coffee 1·0090 Brown malt 1·0109 Parsnips 1·0143 Carrots 1·0171 Yorkshire chicory 1·0191 Black malt 1·0212 Turnips 1·0214 Rye meal 1·0216 English chicory 1·0217 Dandelion root 1·0219 Red beet 1·0221 Foreign chicory 1·0226 Mangold wurzel 1·0235 Maize 1·0253 Bread raspings 1·0263 Assuming the gravity of the pure coffee infusion to be 1·0086, and that of chicory to be 1·0206, the approximate percentage of coffee, C, in a mixture, can be obtained by means of the following equation, in which D represents the density of the infusion:-- C = (1·00(1·020 - D)) / 12. This was tested by mixing equal parts of coffee and chicory, and taking the specific gravity of the infusion; it was 1·01408, indicating the presence of 49 per cent. of coffee. Some idea of the amount of foreign admixture (especially chicory) in ground roasted coffee may be formed from the tinctorial power of the sample. It has already been mentioned that coffee imparts much less colour to water than do most roasted grains and roots. The table below shows the weights of various roasted substances which must be dissolved in 2·000 parts of water in order to produce an equal degree of colour:[11]--