NOVUM ORGANUM. 365 



perfect terrestrial animals, by cutting out the fcetus from the womb ; 

 except as we may take advantage of abortions, animals killed in 

 hunting, and the like. There should, therefore, be kept up a sort of 

 strict vigil over Nature, as being more easily observed by night than 

 by day. For these contemplations may be considered as night- 

 watches, on account of the smallness ol our light nnd its continual 

 employment. 



And the same should be tried in the case of inanimate things, as 

 we have done ourselves in inquiring into the expansion of liquids by 

 fire. For there is one mode of expansion in water, another in wine, 

 another in vinegar, another in verjuice, and quite another in milk and 

 oil ; as it was easy to see by boiling them over a gentle fire, and in a 

 glass vessel, in which the whole actions might be clearly distinguished. 

 But we touch lightly on these things, intending to discourse upon 

 them more fully and exactly when we come to the discovery of the 

 Latent Process of things. For it must always be borne in mind that 

 in this place we are not treating of things themselves, but merely 

 adducing examples. 



xlii. Among Prerogative Instances we shall put in the nineteenth 

 place, Supplementary or Instances of Substitution, which we also call 

 Instances of Refuge. They are those which supply information where 

 the sense is entirely at fault, and in which we therefore take refuge 

 when appropriate Instances cannot be had. Now this substitution 

 takes place in two ways, either by gradation or by analogy. For 

 example : there is no medium discovered which can entirely prevent 

 the magnet from attracting iron. Gold when interposed does not do 

 so, nor yet silver, stone, glass, wood, water, oil, cloth, or fibrous bodies, 

 air, flame, c. Hut yet by accurate tests some medium may perhaps 

 be found to deaden its virtue more than anything else, that is to say, 

 comparatively and in some degree; thus it may be found that the 

 magnet does not attract iron through a thick lump of gold as well as 

 through an equal space of air, or through a mass of ignited silver as 

 well as through a mass of equal size when cold, and so in other cases. 

 For we have not made trial of these things ourselves, but it is 

 sufficient to propose it by way of example. In like manner no body 

 is found with us which is not susceptible of heat when brought near 

 the fire. And yet air contracts heat far more quickly than stone. 

 And such is the substitution which takes place by degrees. 



Substitution by analogy is unquestionably useful, but it is less sure, 

 and must therefore be applied with some discretion. It is used when 

 things not perceptible to the sense are brought within its reach, not 

 by perceptible operations of the imperceptible body itself, but by the 

 contemplation of some cognate body which is perceptible. For 

 example, let inquiry be made concerning the Mixture of Spirits, 

 which are invisible bodies. There seems to be a certain relationship 

 between bodies and what serves as their food or aliment. Now the 

 food of flame seems to be oil and fatty matters ; of air, water and 

 watery matters ; for flame multiplies itself over the exhalation of oil, 

 and air over the vapours of water. We must therefore look to the 



