After visiting the private quarters of the Forbidden City yesterday afternoon, I was, not surprisingly, curious as to how they have been so beautifully and atmospherically well preserved in a culture which has not much cared for its historic past and regards conservation as a process of progressive renewal.   They are the six private palaces north of The Hall of Mental Cultivation (Yangxindian):  the Palace of Eternal Longevity;  the Palace of the Queen Consort;  the Palace for Gathering Elegance;  the Palace of the Supreme Pole;  the Palace of Eternal Spring;  and the Palace of Eternal Happiness.   They survive more or less as constructed in the Ming dynasty, partly because they are not generally open, but only on special application.