26 PHILOSOPHICAL TRANSACTIONS. [ANNO \7Q6. 



The phenomena which heat produces on the solution of cobalt in muriatic or 

 nitro-muriatic acid, called sympathetic ink, have long engaged the attention of 

 chemists and others, but as yet great difficulties have occurred whenever an expla- 

 nation has been attempted. There can be no foundation for the idea which some 

 have had, that the green colour, which characters traced with this solution on 

 paper assume when heated, is caused by a temporary crystallization of the salt, and 

 the disappearance of the colour by a subsequent degree of deliquescence ; because 

 any quantity of the liquid becomes green when heated. 



The effects caused by heat on the sulphuric solution of molybdic acid have there- 

 fore induced me to suspect a similar cause in the muriatic solution of cobalt ; and I 

 believe that heat and cold in like manner causes a temporary difference to take 

 place in the proportions of oxygen existing in the acid menstruum and the oxyde ; 

 and this is the more confirmed when the acid is expelled by too great a degree of 

 heat, for then the changes of colour are no longer to be observed. Heat, it is 

 well known, assists the combination of oxygen with the metals, but I do not be- 

 lieve that the above-mentioned alternate effects of heat and cold have been as yet 

 investigated. It is probable that these are not confined to the two instances which 

 have been adduced, though in other solutions they may not be so apparent. The 

 subject is certainly curious, and worthy of the attention of chemists, as it would re- 

 flect much light on the solutions of metals in general. 



When the sulphuric or muriatic solutions of the molybdic acid are saturated with 

 pot-ash or soda, they assume a very deep blue colour at the moment of saturation. 

 The molybdaena is not however precipitated in the form of the blue oxyde, but for 

 the greater part remains combined with the acid menstruum and the alkali, and 

 thus forms a triple salt in solution, which differs considerably from another triple 

 salt, which is slowly precipitated at the time of saturation in the form of a white 

 flocculent matter, and is composed of the same 3 ingredients, but contains the 

 oxyde in the largest proportion. Sometimes a 4th ingredient becomes added to the 

 last mentioned white precipitate ; for when iron is present in the sulphuric or 

 muriatic solutions, it is precipitated by pot-ash or soda intimately combined with 

 the other ingredients, and appears to render the decomposition of the precipitate 

 very difficult. Though the triple salt which is in solution will pass many folds of 

 paper without leaving any residuum, yet it is not permanent ; for by repeated eva- 

 porations, the neutral salt resulting from the combination of the acid menstruum 

 and the alkali becomes crystallized, and a white flocculent matter is separated, which 

 does not contain iron like that which was precipitated when the acid solution was 

 saturated with the alkali, but can be converted into the yellow molybdic acid by be- 

 ing distilled with nitric acid, which takes from it the small portion of the acid men- 

 struum and the alkali required to constitute the triple salt. 



It has already been observed, that nitric acid has no effect when immediately 

 digested on molybdic acid, but I have found it otherwise when a 3d substance was 

 present ; and the effects were nearly the same whether this substance was a metal 



