Cold air  shrinks the extremities of the exterior fibers of our bodies, and that increases their compression and favors the return of blood from the extremities towards the heart. It decreases the length of these same fibers  ; thereby further increasing their strength. Warm air on the contrary relaxes the extremities of the fibers and lengthens them ; it thus reduces their strength and compression.
People therefore have more vigor in cold climates. The action of the heart and the reaction of the extremities of the fibers work better, the fluids are in better balance, the blood is more strongly propelled toward the heart, and reciprocally the heart has more strength. This greater force must produce many effects : for example, more confidence in oneself, in other words more courage ; more awareness of one’s superiority, in other words less desire for vengeance ; more sense of security, in other words more candor, fewer suspicions, less manoeuvering and guile. In short, it must make for very different characters. Put a man in a warm, closed space : he will suffer, for the reasons I have just stated, considerable heart failure. If in this circumstance a strenuous act is proposed to him, I think he will be found quite indisposed ; his present weakness will plant discouragement in his soul ; he will fear everything, because he will feel he can do nothing. The peoples of warm countries are timid, as are the aged ; those of cold countries are courageous, as are the young. If we consider the last wars,  which are the ones we have most readily in view, and in which we can more easily see certain slight effects imperceptible from afar, we will be quite aware that peoples of the north transported into southern countries  have not performed such great feats there as their compatriots who, fighting in their own climate, benefit from their full courage.
Because of the strength of the fibers of peoples of the north, the coarser juices are extracted from food. From this two things result : first, the parts of the chyle or lymph can with their large surface be more easily applied to the fibers and nourish them ; the other is that they are less able with their coarseness to give a certain subtlety to the nervous juice. These peoples will therefore have large bodies and limited energy.
Each of the nerves which come from every direction into the tissue of our skin makes a bundle of nerves ; ordinarily it is not the entire nerve which is stimulated, but only an infinitely small part of it. In warm countries, where the skin tissue is slack, the nerve endings are expanded and exposed to the smallest movement of the slightest objects. In cold countries, the skin tissue is tight and the papillæ compressed, the small tufts are more or less paralyzed, and sensation can reach the brain only when it is extremely strong and comes from the entire nerve. But it is on an infinite number of small sensations that imagination, taste, sensitivity, and energy depend.
I have observed the outer tissue of a sheep’s tongue, in the spot where it appears to the naked eye covered with papillæ. With a microscope I have seen small hairs, or a sort of down, on these papillæ ; between the papillæ were pyramids that were shaped at their ends into something like small paintbrushes. It seems quite likely that these pyramids are the principal organ of taste.
I had half of this tongue frozen, and found the papillæ considerably reduced to the naked eye ; several rows of papillæ had even withdrawn into their sheaths. I examined its tissue with the microscope and no longer saw any pyramids. As the tongue thawed, the papillæ seemed to the naked eye to rise up again, and in the microscope the small tufts began to reappear.
This observation confirms what I have said : in cold countries the nerve tufts are less expanded ; they withdraw into their sheaths, where they are shielded from the action of exterior objects. The sensations are therefore les vivid.
In cold countries, people will be largely insensitive to pleasures ; they will be more sensitive in temperate countries, and extremely so in warm countries. As we distinguish climates by degrees of latitude, they could be distinguished, so to speak, by degrees of sensitivity. I have seen the operas of England and Italy : they are the same plays and the same actors, but the same music produces such different effects on the two nations, one being so calm and the other so exalted, that it seems inconceivable.
The same will be true for pain : it is provoked in us by the rending of some fiber in our body. The way the author of nature has made things, this pain is sharper as the disturbance is greater ; now it is evident that the large bodies and coarse fibers of the peoples of the north are less susceptible to disturbance than the delicate fibers of the peoples of warm countries ; the mind is thus less sensitive to pain. You have to flay a Muscovite to make him feel anything.
With this delicacy of people’s organs in warm countries, the mind is supremely moved by anything related to the union of the two sexes ; everything leads to this objective.
In northern climates, even physical love has scarcely the power to make itself strongly felt ; in temperate climates love accompanied by a thousand accessories becomes agreeable through things which at first seem to be the thing itself, and are not yet ; in warmer climates love is loved for itself, it is the sole cause of happiness : it is life.
In southern countries, a delicate and feeble yet sensitive machine indulges in a love which in a seraglio endlessly surges and ebbs, or else in a love which, leaving to women more independence, is exposed to a thousand perturbations. In northern countries a healthy and well-constituted but heavy machine finds its pleasures in whatever can quicken the spirits : hunting, travel, war, wine. In northern climates you will find peoples who have few vices, virtues enough, and much sincerity and candor. Moving in a southerly direction, it is like leaving morality itself behind ; more intense passions will multiply crimes ; everyone will seek to seize over everyone else all the advantages that can favor those same passions. In temperate countries you will see people inconsistent in their manners, and even in their vices and their virtues : the climate there is not of sufficiently determinate quality to fix them.
The heat of the climate can be so excessive that the body will utterly lack strength. At that point the exhaustion will even affect the spirit : no curiosity, no noble enterprise, no generous feeling ; all inclinations will be passive, indolence will make for happiness ; most punishments will be less difficult to bear than action of the mind, and servitude less unbearable than the strength of mind that is required for governing oneself.