_The Double Iodide Solution._--In a note appended to DR. MANSELL'S communication on the calotype (Vol. ix., p. 134.), you state that having lately prepared the double iodide solution according to the formula given by DR. DIAMOND, in which it required 650 grains of iodide of potassium to dissolve a 60-grain precipitate, you were inclined to believe, until you made the experiment yourself, that DR. MANSELL must have made a wrong calculation as to the quantity of iodide of potassium (680 grains) which he stated was sufficient to dissolve a 100-grain precipitate, as the difference appeared so small for a solution more than one-third stronger. The small difference referred to with respect to the quantity of iodide of potassium required, is owing to the amount of water used being in both cases the same. A slight difference in the strength of a solution of iodide of potassium makes a great difference with respect to the quantity of iodide of silver it is capable of dissolving. Thus, if you remove a small proportion of the water from a solution of the double iodide of silver by evaporation, the slight increase of strength which the solution will thereby acquire, will enable it to take up a much larger proportion of iodide of silver than it already contains; and if, on the other hand, you dilute it with a small proportion of water, its diminished strength (unless the solution contains a great excess of iodide of potassium) will cause the precipitation of a large proportion of the iodide of silver. And hence the great variation in the amount of iodide of potassium which is found requisite to form a solution of the double iodide of silver, under the same apparent conditions with regard to the proportions of the other ingredients employed, may be accounted for by the impossibility of _measuring_ off with sufficient accuracy the proper proportion of water. Whenever _exact_ quantities of liquids are required, recourse should always be had to the balance, for no great accuracy can be depended upon by measurement with our ordinary glass measures, even supposing them to be correctly graduated, which is not always the case. J. LEACHMAN. _Dr. Mansell's Process._--DR. MANSELL'S lucid and very practical paper on the calotype process in "N. & Q." must, I am sure, be of the greatest service to photographers in general; and as one of the many I am irresistibly tempted to offer my sincere and hearty {183} thanks to him for the truly valuable hints it contains. If DR. MANSELL will give the rationale of the necessity of not allowing a longer time than absolutely required for the soaking out the now injurious iodide of potassium, set free by the deposit of the iodide of silver; and also, an explanation of the cause of that part of the iodized papers which takes the longest time in drying being weaker than that part which had been more hastily dried, the learned Doctor will still be adding to our present account of obligation to him. HENRY HELE.